The electrochemical properties of Se surface-modified Ru/C catalysts (RuSey/C with y = 0 to 1) and their O-2 reduction characteristics were determined in model studies under well-defined mass transport conditions, combining quantitative differential electrochemical mass spectrometry and double-disk electrode thin-layer flow-cell measurements. Surface characterization of the catalysts including the quantitative evaluation of the active surface area was performed by electrochemical/mass spectrometric (combined H-upd adsorption, preadsorbed CO monolayer oxidation, Cu-upd adsorption/stripping, and RuOx formation) methods. The suitability of these methods for the determination of the active surface area in the high and low Se coverage regime are discussed, and COad stripping is found to be the most relevant method for the present catalysts. The kinetic parameters for the ORR ( activity and selectivity) under quasi-steady-state conditions and their variation with Se modification were evaluated in potentiostatic flow-cell measurements. Modification of Ru/C catalyst by Se improves the O-2 reduction activity and reduces the tendency for H2O2 formation in the technically relevant potential region of 0.6-0.8 V-RHE, but even for the best catalyst compositions a significant (similar to 0.2 VRHE) overpotential for O-2 reduction on the RuSey/C catalysts remains compared to that for the Pt/C catalyst, and we find H2O2 yields of at least 1% at typical cathode operation potentials. Consequences of the relatively high H2O2 yields for membrane/electrode stability in practical applications are discussed.