Brownian principles for homogeneous media fail to explain the effect of viscous cosolvents on reaction rates. We examined the possibility that solvent viscosity in the microenvironment of a probe ${\mathrm{\ensuremath{\eta}}}^{\mathrm{*}}$ (microviscosity) differs from the bulk viscosity \ensuremath{\eta} (macroviscosity). ${\mathrm{\ensuremath{\eta}}}^{\mathrm{*}}$ was derived from the diffusion constant D of water tracers, measured in the presence of viscous cosolvents. We found that (a) ${\mathrm{\ensuremath{\eta}}}^{\mathrm{*}}$\ensuremath{\propto}${\mathrm{\ensuremath{\eta}}}^{\mathrm{\ensuremath{\alpha}}}$, where 0\ensuremath{\le}\ensuremath{\alpha}\ensuremath{\le}1, and (b) the temperature dependence of D is insensitive to the presence of these cosolvents. We propose that viscous aqueous solutions have an inhomogeneous microstructure with waterlike domains.