Aluminum plates contaminated with hydrocarbon containing compounds (lubricants) were treated in an argon, oxygen and hydrogen plasma generated by a capacitively coupled 13.56 MHz rf discharge. Power and gas pressure have been varied. During the process the plasma was monitored by optical emission spectroscopy. The spectral line intensities of several species have been measured as a function of the process duration and of the macroscopic parameters. Typical time constants for the cleaning process have been obtained. In order to determine thicknesses and optical properties of the contamination, spectroscopic ellipsometry was used whereas the removal of the contamination layers was observed by means of in situ kinetic ellipsometry. The determined cleaning rates per discharge power are 0.1 nm/Ws for O-2 and about 0.01 nm/Ws for Ar and H-2 which emphasizes the efficiency of oxygen plasma treatment.