Samples of both n-type and p-type GaInAsP, grown lattice matched on semi-insulating Fe doped (100) InP substrates, with carrier concentrations in the low 10¹⁶ cm⁻³, have been analysed to determine their transport properties. The magnitude of the electron mobility, (μ e), at room temperature decreased from about 4,000cm²V⁻¹s⁻¹ at y = 0 passing through a shallow minimum near y = 0.3. From y = 0.3 to y = 1 μe rose steeply reaching 11,000cm²V⁻¹s⁻¹ at the ternary boundary. The temperature variation of μe showed the presence of alloy or space charge scattering, which was maximum near the centre of the alloy range. Similar results were obtained in p-type material where the hole mobility, (μp), varied from 140cm²V⁻¹s⁻¹ in InP, passed through a minimum of about 70cm²V⁻¹s⁻¹ at y = 0.5 and then increased swiftly towards the ternary boundary at y = 1. The temperature dependence of μp again showed the presence of alloy or space-charge scattering. In order to distinguish between these two mechanisms the pressure coefficient of the direct band-gap, dEo/dP, was first measured as a function of y by observing the movement of the photo-conductive edge. From dEo/dP the pressure variation of the electron effective mass, dm*e/dP , was calculated. By measuring the change in electron mobility with pressure, about 20% to 15 k-bar for samples with y = 0.5, it was possible to establish that alloy scattering rather than space charge scattering was occuring. Similar measurements were made on p-type material where μp was observed to decrease by about 5% in 15 k-bar. This was analysed in detail and again confirmed the presence of alloy scattering. From these results it has been possible to establish the composition dependence of the alloy scattering potential for electrons and holes hence a full range of curves have been generated to predict the variation of μe and μp with temperature, pressure and carrier concentration. Measurements were also made at high electric fields in order to determine the effect of alloy scattering on the temperature dependence of the peak velocity, Vp.