We measured and analyzed the far infrared spectra of two typical III-V semiconductors, GaAs and InP. The measurement was performed by an FTIR at room temperature. Observed features other than the Reststrahlen-band were assigned critical points associated with specific two-phonon combinations, according to the strength, position, and shape of two-phonon densities of states which were calculated by the 11-parameter rigid-ion model and the special-k point method. The critical points are related to van Hove singularities of the density-of-states spectra. We located the critical points in an irreducible wedge of the first Brillouin zone and found that the critical points responsible for most prominent features are located not only at high-symmetry points and lines but also on the quadrangular faces of the irreducible wedge.