Experiments probe the effect of hydrogen co-adsorption on the infrared spectral features of carbon monoxide adsorbed at Pt(335) {Pt(S)-[4(111)X(100)]} under aqueous electrochemical conditions. Using intermediate CO coverages, where it is possible to discern infrared spectral features for CO bound terminally (atop) at edge sites and at terrace sites, the present experiments observe greater alterations in the atop CO population at the step edge compared to the atop CO population on the terrace plane when hydrogen is co-adsorbed under aqueous electrochemical conditions. These findings suggest that hydrogen is preferentially adsorbed at step sites on the Pt(335) surface plane and they coincide with what has been revealed by recent UHV experiments which probed the effect of co-adsorbing oxygen at Pt(335) in the presence of a partial CO monolayer.