Diffusion coefficients for a homologous series of alkylbenzenes and alkylphenones have been measured by the Aris-Taylor open tube method from 30 to 60 degrees C over a wide range in methanol/water and acetonitrile/water compositions (10-100% by volume of organic). The measurements were compared to estimates derived from five of the most common empirical correlations. The errors for methanolic mixtures by the Wilke-Chang, Scheibel, and Lusis-Ratcliff correlations are usually less than 20%. The Scheibel, Wilke-Chang, and Hayduk-Laudie correlations work better than others for acetonitrile/water mixtures. Overall, the Scheibel correlation shows the smallest errors, and we recommend its use to that of the more widely used Wilke-Chang method for the systems studied here. We have also developed fitting equations for estimating viscosity so that the diffusion coefficients can be easily estimated.