The ideal way to compare electrocatalysts’ inherent activity, is to compare the turnover frequency per active site (TOF), while eliminating the contribution from the electrochemical surface area (ECSA) of electrocatalysts.(1) However, It is difficult to directly evaluate the TOF, especially for complex nano-scale electrocatalysts in complex electrochemical environments. A more common way to evaluate TOF is via normalization of the current density by the ECSA under relevant electrochemical environments. Accurate estimation of ECSA is thus essential for estimating the activity and understanding the electrocatalyst kinetics. Taking the fundamental hydrogen reduction and oxidation reactions (HER/HOR) as an example, the exchange current normalized by the ECSA is used to estimate the intrinsic kinetics of electrocatalysts. The ECSA of Pt surfaces is conventionally characterized by conducting underpotential deposition/adsorption of hydrogen (HUPD).(2) However, the HUPD integrated area of the carbon supported Pt (Pt/C) nanoparticles and Pt polycrystalline (Pt(pc)) obtained in alkaline is smaller than in acid.(3) In spite of the apparent discrepancy, both the HUPD area in acid and alkaline were used as the ECSA of Pt surfaces when evaluating the inherent HER/HOR activity.(4-6) It is therefore fundamentally important to understand the cause of the lower HUPD area in alkaline than in acid, so as to justify the ECSA evaluation method. Herein, we show with experimental evidence (Figure 1) that the difference of the HUPD area between in acid and alkaline may be explained by the non-specific adsorption of the alkali metal cations (AM+) selectively onto the stepped Pt surfaces.(7) The cation effect is absent for the Pt (111) surface free of defects, and thus its HUPD area remains constant in the wide pH range of 1~13. Since the adsorbed AM+ participate in the HER/HOR, it is reasonable to adopt the HUPD area in alkaline representing the ECSA for the alkaline HER/HOR. We further show that this argument also applies for bimetallic Pt-alloys. Acknowledgement: This work was supported by the Office of Naval Research (ONR) under award number N000141712608. The authors declare no competing financial interests. References D. Voiry, M. Chhowalla, Y. Gogotsi, N. A. Kotov, Y. Li, R. M. Penner, R. E. Schaak and P. S. Weiss, ACS Nano, 12, 9635 (2018). M. J. J. Ian T. McCrum, J. Phys. Chem. C, 121, 6237 (2017). P. J. Rheinlander, J. Herranz, J. Durst and H. A. Gasteiger, Journal of the Electrochemical Society, 161, F1448 (2014). J. Durst, A. Siebel, C. Simon, F. Hasché, J. Herranz and H. A. Gasteiger, Energy Environ. Sci., 7, 2255 (2014). J. Ohyama, T. Sato, Y. Yamamoto, S. Arai and A. Satsuma, Journal of the American Chemical Society, 135, 8016 (2013). W. Sheng, H. A. Gasteiger and Y. Shao-Horn, Journal of The Electrochemical Society, 157, B1529 (2010). X. Chen, I. T. McCrum, K. A. Schwarz, M. J. Janik and M. T. M. Koper, Angew Chem Int Ed Engl, 56, 15025 (2017). Figure 1