Microviscosity can be probed by measuring the rotational correlation time of a probe molecule using either fluorescence correlation spectroscopy or the linewidths of the probe's electron spin resonance. The friction experienced by a single particle can be thought of as a microscopic viscosity (microviscosity) and should not necessarily agree with the bulk viscosity since it is a measure of the probe's local friction whereas bulk viscosity analogously would be the measure of an infinitely large probe. Both the crowding density and relative size of each co-solute in a mixture will contribute to the measured microviscosity as assessed by altered translational mobility.