Reactions of heavy water, D2O, have been measured with 46 atomic metal cations at room temperature in a helium bath gas at 0.35 Torr using an inductively coupled plasma/selected ion flow tube tandem mass spectrometer. The atomic cations were produced at ca. 5500 K in an ICP source and were allowed to decay radiatively and thermalize by collisions with Ar and He atoms prior to reaction. Rate coefficients and product distributions are reported for the reactions of fourth-row atomic cations from K+ to Se+, of fifth-row atomic cations from Rb+ to Te+ (excluding Tc+), and of sixth-row atomic cations from Cs+ to Bi+. Primary reaction channels were observed leading to O-atom transfer, OD transfer, and D2O addition. O-Atom transfer occurs almost exclusively (>= 90%) in the reactions with most early transition-metal cations (Sc+, Ti+, V+, Y+, Zr+, Nb+, Mo+, Hf+, Ta+, and W+) and to a minor extent (10%) with one main-group cation (As+). OD transfer is observed to occur only with three cations (Sr+, Ba+, and La+). Other cations, including most late transition and main-group cations, were observed to react with D2O exclusively and slowly by D2O addition or not at all. O-Atom transfer proceeds with rate coefficients in the range of 8.1 x 10(-3) (As+) to 9.5 x 10(-10) (Y+) cm(3) molecule(-1) s(-1) and with efficiencies below 0.1 and even below 0.01 for the fourth-row atomic cations V+ (0.0032) and As+ (0.0036). These low efficiencies can be understood in terms of the change in spin required to proceed from the reactant to the product potential energy surfaces. Higher order reactions are also measured. The primary products, NbO+, TaO+, MoO+, and WO+, are observed to react further with D2O by O-atom transfer, and ZrO+ and HfO+ react further through OD group abstraction. Up to five D2O molecules were observed to add sequentially to selected M+ and MO+ as well as MO2+ cations and four to MO2D+. Equilibrium measurements for sequential D2O addition to M+ are also reported. The periodic variation in the efficiency (klk,) of the first addition of D2O appears to be similar to the periodic variation in the standard free energy (Delta G degrees) of hydration.