Infrared laser excitation of partially deuterated methanes (CH3D and CH2D2) in a molecular beam is used to control their dissociative chemisorption on a Pt(111) single crystal and to determine the quantum state-resolved dissociation probabilities. The exclusive detection of C-H cleavage products adsorbed on the pt(111) surface by infrared absorption reflection spectroscopy indicates strong bond selectivity for both methane isotopologues upon C-H stretch excitation. Furthermore, the dissociative chemisorption of both methane isotopologues is observed to be mode-specific. Excitation-of symmetric C-H stretch modes produces a stronger reactivity increase than excitation of Me antisymmetric C-H stretch modes, whereas bend overtone excitation has a weaker effect on reactivity. The observed mode specificity and bond selectivity are rationalized by the sudden vector projection model in terms of the overlap of the reactant's normal mode vectors with the reaction coordinate at the transition state.