The emission of MCs+ secondary ions (M designates the analyte species) from TiOx (0.2 <= x <= 2) and GeO. (0.001 <= x <= 0.8) films under Cs+ bombardment was examined. The relative calibration factors of OCs+/TiCs+ and OCs+/GeCs+ were determined and were found to depend pronouncedly on the O/Ti and O/Ge atomic concentration ratios. Specifically, with increasing oxygen content OCs+ ions form much more efficiently (as compared to TiCs+ or GeCs+ ions), an enhancement amounting to more than a factor of 10 for the highest oxygen concentrations. Concurrently, the formation of TiOCs+ or GeOCs+ ions increases drastically. For both oxide systems, an empirical relation for the oxygen-concentration dependence of the relative calibration factors could be established. (c) 2006 Published by Elsevier B.V.