Thermal deformation of surface corrugations fabricated on InGaAsP crystals is investigated in H2 ambient and in the temperature range of 555-620°C to analyze the deformation mechanism. Deformation rate dependence on temperature, corrugation pitch and crystal composition is studied. Surface migration constants and temperature coefficient of In are estimated. The effect of PH3 addition into the ambient on the deformation speed is investigated for various crystal compositions. PH3 addition is remarkably effective in preventing thermal deformation even in InGaAs corrugations which do not contain phosphorus.