The rate constant k(1)(He) of the association reaction Ca + O-2 + He --> CaO2 + He has been measured in the temperature range from 303 to 986 K. The calcium atoms were thermally generated in a fast-flow reactor, and their decay as a function of the added molecular oxygen concentration was followed by atomic absorption spectroscopy. The third-order character having been established at 303 and 734 K, the expression k(1)(He) = (1.5 +/- 0.2) x 10(-28) exp(-10.2 +/- 0.4 kJ mol(-1)/RT) cm(6) molecule(-2) s(-1) has been derived. The bond energy of the CaO2 superoxide reaction product has been estimated on the basis of a simplified RRKM calculation and compared with calculated values from ab initio calculations.