More than 30 years have passed since Congress banned a broad range of synthetic compounds called PCBs. Yet 2.65 million cubic yards of mud on the bottom of the Hudson River remain contaminated with the chemicals, which are considered neurotoxins and probable human carcinogens.

Since 2002, General Electric has been under federal order to clean approximately 40 miles of the Hudson where its factories discharged PCBs. Preliminary site clearing for the huge project began last week, but actual dredging will not start until 2009 at the earliest.

Here, on another PCB-contaminated river about 60 miles to the southeast in Massachusetts, G.E. has made more progress, albeit haltingly.

Working with the federal government, the company completed the cleanup of a two-mile stretch of the Housatonic River late last year, scooping out the heaviest concentrations of the industrial chemicals. While much remains to be done along the remaining 100 miles or more of the Housatonic, its most heavily contaminated section is now cleaner than it has been in a long time.

The two projects differ in scale and expense as much as the two rivers differ in depth and breadth, and each presents unique engineering challenges.

The Hudson project covers 40 miles of the broad river from which the 2.65 million cubic yards of contaminated sediment will have to be dredged, at an estimated cost of $700 million.

By contrast, the first phase of the cleanup of the Housatonic, a far gentler river, had a price tag of $250 million and involved 110,000 cubic yards of mud.

Still, the Housatonic cleanup dragged on for more than six years, twice the expected time. And there is no timetable or cost estimate yet for decontaminating the river's lower reaches.

Based on their experience on the Housatonic, scientists, G.E. officials, government authorities and people who live in the local communities affected by PCBs all agree that work on the Hudson, whenever it starts, is likely to take far longer than expected and will run into more technological obstacles than anyone anticipates.

Almost certainly, the level of public suspicion and mistrust surrounding these projects will not soon subside. Peter L. deFur, an environmental consultant who advises local groups concerned about the Housatonic cleanup, said that although the company had belatedly been doing a ''pretty good'' job on the river, the people who live along the Hudson should be warned that ''G.E. has been absolutely the opposite of a responsible party.''

Stephen D. Ramsey, G.E.'s vice president for corporate environmental programs, who is overseeing both cleanups, said the company had proved its good faith by spending more than a billion dollars studying and cleaning up PCBs, mostly in these two rivers.

But he said there is little that G.E. can do to change perceptions ''except to be very clear with the public about what is the status of the project, what people can expect to see and what are the issues we are encountering as we go along.''

In the last century, G.E. discovered many profitable ways of using PCBs, or polychlorinated biphenyls, a slippery yellowish goo as thick as motor oil that can resist heat without catching fire. G.E. used vast quantities at its Hudson Falls and Fort Edward factories on the Hudson to build electrical capacitors. At its sprawling plant here on the Housatonic, G.E. used them in transformers.

After the chemicals were banned in 1976, the company agreed to clean up its properties. But it argued that the vast quantities of PCBs that had been discharged into the rivers would break down on their own or be harmlessly buried under a protective layer of silt. At first the federal Environmental Protection Agency agreed.

As scientific understanding advanced, however, the agency decided to take a second look. Even nestled in the river mud, the chemicals were being ingested by worms and other invertebrates, which were in turn eaten by small fish, and those were eaten by larger ones, which eventually were consumed by people.

The only ways to solve the problem, the E.P.A. decided in the late 1980s, were to remove or cap the PCBs.

G.E. spent millions of dollars on advertising trying to convince local residents that dredging was worse than doing nothing, but the company eventually accepted a plan to clean more than 40 miles of the Hudson, from Fort Edward to the Troy dam, in two phases.

The first phase, a kind of trial run to last one year, was originally set to begin in 2006 but is now set for 2009. The final phase is scheduled to last five years.

The Housatonic cleanup has a more compact history. The 254-acre G.E. plant in Pittsfield used PCBs from 1932 until they were banned. Storm water runoff from the plant carried the chemicals into the Housatonic, where they spread downstream, contaminating more than 100 miles of river in Massachusetts and Connecticut.

In 1999, G.E. agreed to a $250 million settlement to clean a highly contaminated half-mile portion of the river that ran through the company's property in Pittsfield. Because the Housatonic in that area is only a few feet deep and a few yards wide, G.E. could, in essence, dry-clean the river.