Guided ion beam mass spectrometry is used to examine the kinetic energy dependence of reactions of the second row transition metal cations, Y+, Zr+, Nb+, and Mo+, with molecular hydrogen and its isotopologues. By using a meter long flow tube ion source, we are able to create Zr+, Nb+, and Mof ions that are believed to be in their electronic ground state terms and primarily in the lowest spin-orbit levels and Y+ mostly in its ground state. Corresponding state-specific reaction cross sections are obtained. Analysis of the cross section data yields 0 K bond dissociation energies of D-0(Y+-H) = 2.65 +/- 0.08 eV, D-0(Zr+-H) = 2.26 +/- 0.08 eV, D-0(Nb+-H) = 2.28 +/- 0.07 eV, and D-0(Mo+-H) 1.72 +/- 0.06 eV. This thermochemistry is compared with theoretical calculations and previous experimental measurements. Results for the HD reactions indicate that Y+(D-3) reacts via a statistical mechanism, Zr+(F-4), Nb+(D-5), and Mo+(S-6) react via largely statistical mechanisms, and Y+(S-1) shows complex behavior. The reaction mechanisms and reactivity differences among these ions are explained by using simple molecular orbital concepts and by referring to potential energy surfaces calculated by Das and Balasubramanian.