We have previously calculated intervalley scattering (IVS) times for GaAs, resulting in good agreement with various experiments, although some details are still under discussion. In this paper, we consider a semiconductor alloy like Al1-xGaxAs, where scattering due to disorder competes with phonon-assisted intervalley scattering. We have calculated the scattering times from the (Gamma) point in indirect Al1-xGaxAs (x > 0.4) to the satellite valleys at L and X using a second-order perturbational approach. Other methods have also been investigated. We find that disorder-induced scattering is about twice as effective as phonon-assisted scattering, even at room temperature. We also calculate the lifetime broadenings caused by these processes and compare them to experimental broadenings of the E0 gap obtained by resonant Raman scattering and spectroscopic ellipsometry. IVS times in InP from the (Gamma) valley to side valleys around L and X are given as a function of energy at 0 and 300 K as well as the return times from the L and X points to the (Gamma) valley from 0 to 400 K. Our results suggest that the strength of the electron-phonon interaction in InP is similar to that in GaAs. Therefore, the IVS times in InP can also be obtained from those of GaAs by a simple extrapolation allowing for the different position of the satellite valleys. Conclusions about IVS times in Al1-xGaxAs, however, cannot be drawn from data on GaAs, because most IVS in Al1-xGaxAs is due to alloy scattering, which does not occur in GaAs.