At anodically polarized Pt electrodes in aqueous H2SO4 solution, Cl- and Br- ions are adsorbed competitively with the oxygen species that is deposited between 0.8 and 1.1 V, RHE, forming the initial stage of anodic oxide-film generation at Pt anodes. The Cl- adsorption leads to selective and progressive blocking of the first 50% of coverage by O species at the Pt surface with increasing Cl- concentration (commencing at 10(-7) M) while relatively little effect is seen on the place-exchanged Pt/O --> O/Pt lattice that is formed beyond 1.10 V, except at Cl- concentrations greater than 10(-1) M, where evolution of Cl-2 commences and hence interferes. The states of co-adsorbed Cl and O-containing species, and their relative surface coverages, determine the nature of the electrocatalytic surface on which anodic Cl-2 evolution takes place. How this selective blocking behavior arises has not previously been well understood. In the present paper, complementary applications of the electrochemical quartz crystal nanobalance (EQCN) technique with cyclic voltammetry are brought to bear on this matter and provide new results at high levels of sensitivity and reproducibility which help to elucidate the competitive and selective chemisorption effects of Cl- at Pt anodes. The results obtained with Cl- ions exhibit some peculiarities which comparative experiments on Br- adsorption help to rationalize.