- An approach to the calculation of disorder scattering in ternary semiconductor alloys is proposed. It is based on the Hellman-Feynman theorem of quantum mechanics and enables the calculation of alloy scattering relaxation times provided that the dependences of the band edges and the effective masses on the alloy composition are known. The result obtained incorporates nonparabolicity effects not only via the electron spectrum but also the p -wave mixing matrix element renormalization. As an example the calculation of the alloy scattering relaxation times for three main valleys in GaAlAs is considered. It is shown that the model existing in the literature strongly overestimates the alloy relaxation rate in the Γ-valley at high energies and gives an incorrect relation between the alloy relaxation rates in Γ, X and L valleys.