A simplified formulation is presented to deal with interfacial stability problems with mass and heat transfer. For Rayleigh–Taylor stability problems of a liquid‐vapor system, it is found that the effect of mass and heat transfer tends to enhance the stability of the system when the vapor is hotter than the liquid, although the classical stability criterion is still valid. For Kelvin–Helmholtz stability problems, however, the classical stability criterion is found to be substantially modified due to the effect of mass and heat transfer.