Ga x In 1- x As lattice matched to InP has been shown to have 50% higher mobility at 300 K than GaAs, and therefore is useful as an active layer for FET devices in either microwave or high speed logic applications. Results of Hall mobility characterization of Ga x In 1- x As epitaxial layers are presented showing a maximum measured mobility of μ 300 = 13,800 cm 2/V · s. Analysis of variable temperature Hall effect measurements (4–300 K) conducted on high purity material has allowed separation of the different scattering mechanisms. Results show that alloy scattering reduces the measured mobility by about a factor of two in high purity samples at 77 K and has a small, but noticeable (∼15%) effect at room temperature.