The capsule materials Au, Ag 75 Pd 25 , and Pt, were tested for permeability to C-H-O fluids in a piston-cylinder apparatus at 10 kbar and 900-1100 degrees C. The capsules were embedded in boron nitride. After the experiments, the C-H-O fluids were analyzed by gas chromatography to determine the oxygen content of the capsules. No significant loss of oxygen was observed in any capsule material at temperatures < or =1050 degrees C. At 1100 degrees C, however, which is above the melting point of Au and Ag 75 Pd 25 at 10 kbar, oxygen loss from Pt capsules amounted to between 12 and 80%, over a period of 6 days. These results were confirmed by experiments performed using pure H 2 O as the fluid. Water loss during piston-cylinder experiments is a serious problem. A temperature of about 1050 degrees C represents the upper thermal limit for constant H 2 O-containing systems.