The decomposition of H2O2 to H2O and O-2 is a fundamental chemical reaction catalyzed by platinum nanocatalysts. We recently showed that this is a two-step chemical reaction and that the rate of the overall reaction correlates with particle size for platinum samples subjected to heat. However, the mechanism by which particle size affects the rate of this reaction is not understood. We combined atomic-scale characterization of platinum nanocatalysts subjected to various conditions with multi-variate linear analysis of H2O2 decomposition rate data to determine structure-chemistry/activity relations of platinum nanocatalysts in the decomposition of H2O2. Catalyst particle size, determined through Rietveld analysis, ranged from 8.8 nm to 41 nm. Our results show that the effect of particle size on the rate of H2O2 decomposition is caused by a correlation of particle size with surface chemisorbed oxygen abundance - quantified as the fraction of catalyst surface covered with chemisorbed oxygen (theta). Surface theta has a nearly first order effect on the rate of H2O2 decomposition because the oxidation of Pt -> Pt(O) is the rate limiting step of the reaction. When the effects of particle size and surface theta are decoupled the effect of particle size becomes trivial. The approach applied in this study provides a valuable methodology that can be used to deconvolute the effect of correlated variables in many other catalytic chemical reactions of scientific and technological interest. (C) 2019 Elsevier Inc. All rights reserved.