Self-terminating electrodeposition was used to grow ultrathin Pt overlayers on 111 textured Au thin films. The Pt thickness was digitally controlled by pulsed potential deposition that enabled the influence of overlayer thickness on electrocataytic reactions, such as methanol and formic acid oxidation, to be examined. Bimetallic and ensemble effects associated with submonolayer coverage of Pt on Au yield enhanced catalysis. For films grown using one deposition pulse, the peak rate of CH3OH oxidation was enhanced by a factor of 4 relative to bulk Pt. The overlayer consisted of 2 nm diameter monolayer Pt islands that covered 75% of the surface; however, voltammetric cycling resulted in a loss of the enhanced activity associated with the as-deposited submonolayer films. For thicker Pt films, the electrocatalytic activity decreased monotonically with thickness until bulk Pt behavior was obtained beyond three monolayers. For HCOOH oxidation improvements in the Pt area, normalized activity in excess of a 100-fold were observed for submonolayer Pt films. The performance improved with voltammetric cycling as a result of a combination of Pt dissolution, Au segregation, and Pt–Au alloy formation. The maximum activity was associated with fractional surface coverage between 0.28 and 0.21, although the films were subject to a deactivation process at longer times related to a diffusional process. Bulk Pt behavior for formic acid oxidation was observed for Pt films greater than three monolayers in thickness.