The main goal of this study was to develop and test a capsule assembly for use in piston-cylinder experiments where oxygen fugacity could be controlled in the vicinity of the QFM buffer without H O loss or carbon contamination of the sample material. The assembly consists of an outer Pt-capsule containing a solid buffer (Ni-NiO or Co-CoO) plus H O and an inner AuPd-capsule, containing the sample, H O and a Pt-wire. No H O loss is observed from the sample, even after 48 h, but a slight increase in H O content is found in longer runs due to oxygen and hydrogen diffusion into the AuPd-capsule. Oxygen fugacity of runs in equilibrium with the Ni-NiO (NNO) and Co-CoO (CoCO) buffers was measured by analyzing Fe dissolved in the Pt-wire and in the AuPd-capsule. The second method gives values that are in good agreement with established buffer values, whereas results from the first method are one half to one log units higher than the established values.