The upward diffusion cloud chamber was used to study the homogeneous nucleation of the MeOH–H2O and n-PrOH–H2O mixtures over a wide range of water activities (i.e., relative humidities). The results are compared with the predictions of theory. Good agreement between the experimental results and the classical theory of homogeneous nucleation was found for alcohol rich mixtures. However, at high water concentrations, the experiments require much more alcohol than theoretically predicted. This is the same conclusion as was obtained previously for the EtOH–H2O mixture. It is shown that this disagreement can be explained by the fact that, for the alcohol–water systems, the use of macroscopic surface tension is not relevant to binary nucleation experiments.