This paper discusses the proper measure of the electrochemically active area (ECA) of carbon supported Pt catalyst in polymer electrolyte fuel cells employing in situ cyclic voltammetry. The charges of the hydrogen underpotential deposition (H-upd) and CO stripping peaks obtained in situ are compared, and the influence of operation temperature (25-80 degrees C) and relative humidity (40%-90%) is discussed. The results show that the charges of the Hupd decrease with rising temperature, while the corresponding charges of the CO stripping peak are essentially independent of temperature, at least at high relative humidity. The unexpectedly small Hupd charges are explained by the significant overlap with the hydrogen evolution reaction in a fuel cell at elevated temperatures. According to our results, it is proposed that a more reliable value of Pt ECA is estimated from the CO stripping charge. However, with decreasing humidity the charges of both Hupd and CO stripping peaks decrease, which is probably an effect of increasing blockage of Pt active sites by hydrophobic domains in the electrode ionomer. Some implications of varying cell conditions on the estimated Pt ECA and its correlation with fuel cell activity are discussed in an example from a fuel cell degradation test. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3494220] All rights reserved.