The adsorption and oxidation reactions of squaric acid (SQA) at platinum single crystal electrodes with basal orientations have been studied by combining voltammetric and in situ FTIR experiments. Squaric acid is adsorbed dissociatively at these platinum surfaces, giving rise to dense CO adlayers in a potential-dependent structure-sensitive process. The resulting CO adlayers have been characterized spectroscopically and found similar to those formed at each electrode surface after dosing CO from a saturated solution. The main reaction path in the SQA oxidation mechanism involves the formation and subsequent oxidation of adsorbed CO to carbon dioxide, which is the main reaction product at potentials higher than 1.10 V. Both the adsorption of squarate anion and the formation of cyclobutantetrone as an intermediate product are limited by the formation of the CO adlayer. These two species have been detected in a significant amount only in the case of Pt(111), for which the rate of CO formation from SQA between 0.7 and 1.0V is the lowest among the basal orientations.