Gas-vapor nucleation in liquid solutions is considered. It is shown that for nucleation to occur in a multicomponent solution a nucleus must be created of at least a certain radius. Consideration is then restricted to weak solutions of gas dissolved in liquids with the gas allowed to behave nonideally. A generalized Kelvin equation is derived which relates the partial pressure of the solvent vapor inside a nucleus to the properties of the surrounding solution. It is shown how, for a given temperature, the pressure at which nucleation occurs in a gas-liquid solution is increased as the gas concentration is increased.