Electronic transport properties of strain-relaxed Si-doped InAsyP1-y layers with arsenic mole fractions between y=0.05 and y=0.50 were studied. All layers were grown on semi-insulating InP substrates by solid source molecular beam epitaxy using intermediate InAsyP1-y step-graded buffers to reduce dislocation density. Variable magnetic field (0–8.5 T) Hall effect measurements in conjunction with quantitative mobility spectrum analysis in the temperature range of 25–300 K were used to extract individual carrier mobilities, densities, and donor ionization energy as a function of temperature and alloy composition. The low field mobility is calculated by taking into account various scattering mechanisms, and these results are compared with the experimental results. At a constant electron carrier concentration of ∼2×1016 cm-3, the 300 K carrier mobility increases from 2856 to 5507 cm2/V s with increasing arsenic mole fraction from 0.05 to 0.50. The experimental mobilities are in close agreement with the theoretical results using various scattering mechanisms. Both optical polar phonon scattering and ionized impurity scattering are important at 300 K while at 100 K, ionized impurity scattering is the limiting process. Alloy scattering is found to be only of second order importance. The Si donor ionization energy was determined to be ȣ- c;2–4 meV for all alloy compositions.