The reactivity of molybdenum atoms and dimers with respect to H-2, N-2, CH4, O-2, N2O, CO2, CO, C2H4, C3H6 (propene), and NH3 in He buffer gas in the pressure range 0.44-8 Torr at room temperature has been investigated by using a flow tube reactor equipped with a laser vaporization source for production of Mo and Mo-2. Detection and monitoring of Mo and Mo-2 is by resonance fluorescence excitation. The performance of the flow tube is investigated over a range of operating conditions by studies of several test reactions of Ti and Cr atoms, for which rate constants have been measured in previous work. Factors that contribute to the uncertainty of rate constants measured by using the laser vaporization flow tube reactor are discussed. Molybdenum atoms and dimers show significant differences in reactivity, in particular with respect to adduct formation reactions with cc-acceptor and Lewis base Ligands. The possible origins of these differences in features of the valence electronic structures of Mo and Mo-2 are discussed, and the role of repulsive interactions in controlling reactivity is emphasized. In view of parallels with coinage metal atoms and dimers, generalizations of reactivity differences between transition metal atoms and dimers are suggested. Analysis of kinetic data for Mo + C2H4 and Mo-2 + NH3 association reactions leads to the following estimates for binding energies of the 1:1 association complexes : Delta E(0) degrees/kcal mol(-1) = 17(-3)(+4) for Mo[C2H4] and 14 +/- 2 for Mo-2[NH3].