A theory of alloy scattering of two‐dimensional electron gas in quaternary III‐V semiconductors is developed by assuming spherically symmetric square scattering potential randomly distributed in the crystal. The theory predicts a temperature‐independent mobility. Electron mobilities have been calculated for two‐dimensional electrons in Ga1−xInxP1−yAsyand Ga1−xInxP1−ySby, with scattering potentials expressed in terms of the differences in the band gaps, the electron affinities, and the electronegativities of the constituent materials.