Simultaneous measurements of both viscosity and light scattering are performed on two critical mixtures. The first one is a triethylamine-water solution, which exhibits a lower consolute point, the second one a methanol-ciclohexane mixture with an upper consolute point. It is found that the singular behavior of viscosity cannot be fitted by a simple power law, nor by a logarithmic one, in the entire range of temperature. The asymptotic behavior, however, tends to become logarithmic as the critical temperature is approached. The simultaneous observation of scattered light allows one to exclude the intervention of spurious processes, like a breaking of correlations because of impurities. In addition, it is shown that the correlation length seems to depend mainly on the reduced temperature, irrespective of the system under examination.