The pure Marangoni and pure evaporation problems are both associated with interfacial instability, and a comparison is made between the two problems in this paper by way of illustrative calculations. In each case, the scaled critical temperature drop against wavenumber is graphed, as is also the scaled growth constant of the disturbance against wavenumber. The comparison shows the similar roles of transverse diffusion and surface curvature in the two problems. Transverse diffusion is seen to play a dual role, and the physics involved in this dual role is explained.