We report on convection experiments with a fluid with Prandtl number Pr = 7 in a Czochralski model system and in annular gaps at normal gravity (1 - g) and under microgravity (mu - g). The results are applicable to oxide and fluoride melts. Thermocapillary flow penetrates fully into the fluid volume under microgravity whereas it separates into a layer-like flow on top of the liquid at normal gravity. This surface tension gradient-driven flow becomes unstable under microgravity and under normal gravity. By frequency analysis we can assign a hydrothermal wave-like structure to this instability for shallow liquid layers under microgravity and for all liquid heights h under normal gravity. The critical temperature differences DeltaT(c) for the onset of the oscillation are independent of h in all cases. Because of the separation mechanism the oscillation period is independent of h under 1 - g and comparable in the annular gap and in the Czochralski system. (C) 2002 Elsevier Science B.V. All rights reserved.