The thermal decomposition of bulk and heteroepitaxial (100) InP surfaces is studied by in situ scanning electron microscopy combined with mass spectrometry. The onset of P evaporation coincides with the In droplet nucleation at about 480 degrees C and the major evaporation of phosphorous commences above 510 degrees C and corresponds to the serious deterioration of the surface. There is no significant difference between bulk and defected (heteroepitaxial InP/GaAs) samples. The relevance to InP technology is discussed.