InGaAsP films grown on InP substrate by solid source molecular beam epitaxy (SSMBE) using a valve phosphorous cracker cell are investigated. It is found that the films grown at flux ratios f(As)/(f(As) +f(p)) from 0.45 to 0.50 show a superior quality. It is also found that As pressure plays a crucial role in the scattering process; for the films grown at higher arsenic beam pressure (BEP), the Hall mobility mu is dominated by impurity scattering, polar phonon scattering and alloy scattering. For the films with high quality, optical scattering and alloy scattering dominate the mobility. The exponent of T for the films grown at low BEP is found to be as high as 2.54, which cannot be explained by impurity scattering alone. It is believed that, in addition to the impurity-related scattering. some defects associated with As vacancies also significantly contribute to the scattering, especially at low temperatures.