In the process of r.f. plasma sintering of Al2O3 at reduced pressure, a number of puzzling observations have been reported. They include higher heating rates and higher maximum temperature of a sample at increased propagation velocities of the sample through the plasma and a sudden temperature drop of approximately 700°C if the sample motion in the plasma is stopped. An analysis of the sintering process indicates that the change of che surface area and of the catalytic properties of the sample surface during sintering seem to be responsible for these observations. The change from an initially negatively charged co an essentially non-charged surface causes a drastic decrease of the heat transfer from the plasma to the sample. A simulation of the plasma sintering process based on a r.f. argon plasma provides a semi-quancicative explanation of all the observed phenomena.