GaInAsP lattice matched to InP is known to have two well‐resolved InP‐like and InGaAs‐like longitudinal optical (LO) phonons over the composition range 0<y<1. Here for the first time the electron Hall mobilities in GaInAsP are calculated by the iterative technique taking the scattering due to the two LO phonons into account. The results show a further downward bowing of the mobilities than those calculated with one‐LO‐phonon model.