Positively charged clusters of V, Nb, Ta, Co, Rh, Ir, and Ni in the size range of 3-26 atoms were generated by laser vaporization in a supersonic nozzle and injected into the ion trap of a Fourier transform ion cyclotron resonance apparatus, where they were studied by exposing them to hydrogen gas. Some clusters experience structural changes during the chemisorption process, and some clusters have multiple stable isomers. The reactivity of positive V, Nb, and Ta clusters toward H\sb2 increases with cluster size, but the reactivity oscillates so that the even clusters can be several orders of magnitude less reactive than their odd neighbors. The reactivity of positive Co, Rh, Ir, and Ni clusters toward H\sb2 varies with the cluster size, but the variation is smooth. In general clusters made of atoms with the same number of valence electrons behave quite similarly. The high reactivity of most positive clusters is attributed to unoccupied d-like states lying near the fermi level--these states are needed to avoid the Pauli repulsion, which creates the activation barrier for the H\sb2 chemisorption.