Using first-principles total-energy calculations, we investigate the structural, electronic and thermodynamic properties of the cubic Sc 1-xInxP semiconducting alloys. The calculations are based on the fullpotential linearized-augmented plane wave (FP-LAPW) method within density functional theory (DFT). The exchange-correlation effect is treated by both local-density approximation (LDA) and generalizedgradient approximation (GGA). In the latter approach, both Perdew-Burke-Ernzerhof (PBE) and EngelVosko (EV) functional of the exchange-correlation energy were used. The effect of atomic composition on structural parameters, band-gap energy, mixing enthalpy and phase diagram was analyzed for x = 0, 0.25, 0.5, 0.75, 1. Lattice constant, bulk modulus, and band-gap energy for zinc-blende Sc1-xIn xP alloys show nonlinear dependence on the aluminium composition x. Deviations of the lattice constant from Vegard's law, and deviations of the bulk modulus and band-gap energy from linear concentration dependence (LCD) were found.