The contribution of the electrocatalyst support to polymer electrolyte membrane (PEM) oxidative degradation in an operating polymer electrolyte fuel cell was investigated. A corrosion-resistant non-carbon catalyst support based on mixed ruthenium and silicon oxides (RuO2-SiO2; RSO) was compared against a benchmark carbon-based support (Vulcan XC 72; C). The rates of in-situ reactive oxygen species (ROS) generation (Pt/C: 9.0 +/- 0.20 x 10(-5) s(-1); Pt/RSO: 5.9 +/- 0.19 x 10(-5) s(-1)) and macroscopic PEM degradation measured ex-situ as the fluoride emission rate (FER; Pt/C: 2.7 +/- 0.32 x 10(-5) ppm cm(-2) s(-1); Pt/RSO: 2.5 +/- 0.31 x 10-5 ppm cm(-2) s(-1)) were significantly lower for platinum supported on RSO than for platinum supported on carbon. There was an excellent correlation between the in-situ ROS generation rate and the FER, thereby confirming the causal relationship between ROS generation and PEM degradation. The lower rate of ROS generation over RSO and Pt/RSO was attributed to a lower net rate of electrochemical H2O2 generation during the oxygen reduction reaction (ORR). Rotating ring-disk electrode experiments confirmed that the net electrochemical H2O2 generation rate on Pt/RSO was about twice lower than that on Pt/C. Kinetic parameters estimated for the ORR supported a direct 4e(-) pathway on both Pt/RSO (with i(k) of 4.5 mAcm(-2), n = 3.97 and a Tafel slope of 64 mVdec(-1)) and Pt/C (with i(k) of 4.1 mAcm(-2), n = 3.93 and a Tafel slope of 68 mVdec(-1)). In conjunction with its high corrosion-resistance, this finding further illustrates the viability of RSO (and analogs such as ruthenium-titanium oxide) as outstanding PEFC electrocatalyst supports. (C) The Author(s) 2016. Published by ECS. All rights reserved.