Using Big Bang models, it is possible to calculate the expected concentration of the isotopes helium-4 (He), helium-3 (He), deuterium (H), and lithium-7 (Li) in the universe as ratios to the amount of ordinary hydrogen. The relative abundances depend on a single parameter, the ratio of photons to baryons. This value can be calculated independently from the detailed structure of CMB fluctuations. The ratios predicted (by mass, not by abundance) are about 0.25 for He:H, about 10 for H:H, about 10 for He:H, and about 10 for Li:H.