The compositional dependence of In1−xGaxAsyP1−y(y= 2.2x) MIS (metal‐insulator‐semiconductor) diodeC‐Vcharacteristics was investigated for entire As fraction range (0⩽y⩽1). As the gate insulator, the pyrolytic Al2O3film was employed. Alln‐type MIS diodes fabricated on InP, In0.76Ga0.24As0.55P0.45and In0.53Ga0.47As showed almost entire capacitance variation between the accumulation and high‐frequency inversion capacitances. The compositional dependence of theC‐Vcurve was clearly seen in its hysteresis and the surface state density distribution. TheC‐Vcurve hysteresis becomes less pronounced as the crystal composition is closer to InGaAs. The botton of the V‐shaped surface state density distribution shifts toward the conduction band edge as As fraction (y) increases. Therefore, the surface state density near the conduction band edge was reduced to 1012/cm2eV, compared to the value 1013/cm2eV at InP surface.