Abstract A highly active inorganic pyrophosphatase from a thermophilic bacterium has been partially purified and studied in some detail. The enzyme was dependent upon magnesium ions for activation and was activated to a lesser extent by cobalt. Neither calcium nor manganese was effective as an activating ion. The pH-activity curve was quite broad and exhibited a distinct plateau in the range from pH 5.5 to pH 9.5. The usual Lineweaver-Burk plot of (S) v versus ( S) did not give a straight line. Plotting (S) 2 v versus ( S) 2 gave a straight line from which K s was calculated to be 5 × 10 −3. Evidence is presented that the formation of a complex metallosubstrate is necessary for the hydrolysis of pyrophosphate by the enzyme. The energy of activation was found to be 21,000 cal./mole in the range from 45 to 70 °C. and 34,400 cal./mole in the range 30–45 °C. Apparent thermodynamic values were calculated from heat-inactivation data. The most unusual of these were the entropies of activation, Δ S ++. For a 20-min. period of inactivation the mean Δ S ++ value was 13.0 cal./mole/deg. After a 40-min. period of inactivation, the Δ S ++ value was 45 cal./mole/deg. The low Δ S ++ values suggest that there may be inherent structural differences in the thermal enzymes which render them less vulnerable to the denaturing effects of heat.