We have investigated the compositional dependence of surface morphology and electronic properties of modulation-doped InxAl1-xAs/InyGa1-yAs structures (0.2 less than or equal to x,y less than or equal to 0.8) on strain-relaxed buffers on GaAs and InP substrates. We have found that modulation-doped In0.70Al0.30As/In0.70Ga0.30As on InP shows a two-dimensional electron-gas mobility of 1.3 X 10(4) cm(2)/Vs (300K) and 9.3 X 10(4)cms (77K) at a sheet carrier concentration of 2.0 X 10(12)cm(-2). We suggest that this material system has a potential for device applications if a high-quality Schottky barrier layer can be fabricated.