The loss in electrocatalytic activity of Pt particles in carbon matrix electrodes has been experimentally and theoretically investigated as a function of Pt particle size. The measurement of the cathodic potentiostatic current transient showed that a decrease in oxygen reduction current due to carboxyl group formation, relative to the oxygen reduction current in the absence of carboxyl group, increased with a decreasing Pt particle size. This relative value is a measure of the loss in specific activity. A model describing the electrocatalytic activity loss has been proposed by introducing a new parameter, characterising the effective dead active area produced by the carboxyl group formation, relative to the total active area free of the carboxyl group. The agreement of the experimentally determined relative current decrease with the calculated relative value of the effective dead active area confirms the model.