Theoretical estimates have been made for the ionisation coefficients of electrons in Ga_1-xIn_xAs_yP_1-y alloys. Two interpolation schemes for the estimation of ionisation coefficients have been presented. In the first scheme the authors have calculated the ionisation coefficients of carriers of the constituent ternary alloys and then used an interpolation formula to find the ionisation coefficient in the quaternary alloy. The alternative approach is first to calculate all the material parameters for the quaternary alloy using the interpolation relation and then use the ternary ionisation coefficient formula with the assumption that the nature of scattering events in quaternary alloys is similar to those of ternary alloys. Theoretical results are found to have good agreement with the experimental data of Osaka and co-workers(1984).