There is controversy about the mechanism of singlet oxygen (/sup 1/O/sub 2/) formation from systems that contain superoxide ion (O/sub 2//sup .-/), halocarbons, and H/sub 2/O. Khan and Corey et al. have proposed that the /sup 1/O/sub 2/ results from the water-induced disproportionation of O/sub 2//sup .-/, while Arudi et al. and Kanofsky have proposed that the /sup 1/O/sub 2/ is a product of the reaction of O/sub 2//sup .-/ with halocarbons, where X is Br or Cl. Several prior studies support the latter hypothesis. The interpretation of past experimental results is difficult, however, because all prior chemiluminescence studies that demonstrate /sup 1/O/sub 2/ production have used either a two-phase system (halocarbon-/sup 2/H/sub 2/O/solid KO/sub 2/) or a three-phase system (halocarbon/H/sub 2/O/solid KO/sub 2/). Via the use of a homogeneous system with acetonitrile as the solvent, they now report that (1) O/sub 2//sup .-/ reacts with a number of halocarbons to produce /sup 1/O/sub 2/, (2) the addition of /sup 2/H/sub 2/O to O/sub 2//sup .-/ in acetonitrile does not produce /sup 1/O/sub 2/, and (3) the addition of /sup 2/H/sub 2/O to halocarbon plus O/sub 2//sup .-/ reactions does not increase the yield of /sup 1/O/sub 2/.