The surface topography of the thermal oxides on InP are shown to blister for growth temperatures somewhat above 600 °C. It appears that the oxide softens sufficiently to allow the trapped phosphorus at the interface to expand, thus causing the surface bubbles to form. The size of the bubbles varies considerably across a wafer and they tend to increase in size with temperature. At 780 °C one bubble had a radius of ≊100 μ. At T≲650 °C the elemental phosphorus at the interface is the source of the P and the underlying substrate remains smooth. Above this temperature the InP substrate provides the P as shown by the pits in the substrate. There appears to be a relationship between the surface roughening and a change in oxide layer composition.