The gas-phase reactions of Pt+ and Au+ with C2H4, C3H6, 1-C4H8, cis-2-C4H8, and trans-2-C4H8 have been carried out under thermal and near-thermal conditions using a SIFT reactor and a drift cell. In all reactions examined here, Pt+ exhibits dehydrogenation as the dominant primary product channel, but elimination of methane and ethene are observed in some systems in small amounts. Evidence for both allylic and vinylic activation of sigma-bonds is observed. Observation of these bimolecular processes allow us to calculate D(Pt+. acetylene) > 49.76 +/- 0.01 kcal/mol, D(Pt+. allene) > 40.75 +/- 0.01 kcal/mol, D(Pt+. propyne) > 39.7 +/- 0.1 kcal/mol and D(Pt+. butadiene) > 29.01 +/- 0.01 kcal/mol. Hydride abstraction is also observed as a minor product channel in the reactions of Pt+ with C3H6 and all three butene isomers. Secondary and tertiary reactions in the Pt+ systems are limited to clustering reactions. Hydride abstraction is the only primary bimolecular pathway observed in the reactions of Au+ with propene as well as the butene isomers. Au+ forms adducts with both C2H4 and C3H6 in primary steps. Follow-on reactions are also observed in the Au+ systems which include both bimolecular and clustering steps. Ion mobility experiments carried out using the drift cell indicate that both Pt+ and Au+ are in their ground states in the reactions described here. However, we show evidence that an Au+ excited state can be populated under the appropriate ionizing conditions. Reduced zero-field mobilities for ground-state Pt+ and Au+ have been determined to be 20.6 +/- 0.6 and 19.5 +/- 0.5 cm(2)/V . s, respectively.