We have investigated evaporative instability by a nonlinear analysis. This problem has some similarities to the solidification instability of a pure substance or the electrodepostion instability of a metallic ion. There are several ways to view how one might conduct an evaporation experiment and in one of these the input variables are the depths of fluid layers, the temperature difference across the system and the pressure at the top plate. Once these are fixed, the outputs are the bifurcation nature and the evaporation and heat transfer rates. We present calculations which tell us that when the instability sets in the branching can be either a forward or backward pitchfork but the evaporation rates are always increased. In this regard the problem is quite unlike solidification or electrodeposition. We will also present results that take into account experimental complications such as side wall effects and buoyancy.