We studied degradation mechanism of platinum catalysts films of ultralow loading on titanium nitride, highly oriented pyrolytic graphite and Sigradur G glassy carbon substrates. The films were prepared by magnetron sputtering with platinum amount nominally corresponding to 1, 5 and 10 layers. The catalyst stability was tested by potential cycling in perchloric acid. We find that dependence of catalyst stability on the substrate type is very much function of catalyst loading. This is a result of different catalyst degradation mechanisms. For samples with 1 platinum layer we observe that three dimensional crystallites are formed from two dimensional initial nanoislands and degradation mechanism follows power laws derived for three dimensional nanoparticle growth; for samples with 5 platinum layers we observe formation of interconnected network of two dimensional islands. The electrochemical surface area loss does not depend much on the substrate type and the degradation mechanism follows power law developed for two dimensional particle growth. Catalyst films made by deposition of 10 platinum layers are continuous films on all three studied substrates and after potential cycling, the films stay continuous with small difference of the film stability for studied substrates. For studied platinum loadings carbon substrates performed better than titanium nitride substrate. (C) 2016 The Electrochemical Society. All rights reserved.