Partially deuterated polystyrenes (PS-d(3) and PS-d(5)) were examined using TOF-SIMS and regression analysis. Intramolecular H/D exchange was observed for both small fragment ions (C6H7+ etc.) and for main chain fragments in the mass range from 500 to 3000 Da. Depending on the type fragment, either one or two exchanges occur. Facile H/D mixing between the polymer backbone and the phenyl ring must occur during formation of all types of fragments. Comparison of the results from regression analysis showed that for PS-d(3) hydrogen abstraction by the chain backbone occurred much more readily than the corresponding deuterium abstraction for PS-d(5).