The reactions of O3with acetylene, methylglyoxal and a series of aromatic hydrocarbons were studied at 297 ± 2°K. Rate constants k2were obtained from the O3decay rates in the presence of excess reactant and were determined to be (k2× 1020cm3molecule‐1sec‐1): acetylene 3.8 ± 0.6; methylglyoxal 0.11 ± 0.05; benzene 0.007 ± 0.004; toluene 0.015 ± 0.008; o‐xylene 0.07 ± 0.02; m‐xylene 0.06 ± 0.02; p‐xylene 0.04 ± 0.02; 1,2,3‐trimethylbenzene 0.16 + 0.04; 1,2,4‐trimethylbenzene 0.13 ± 0.03; 1,3,5‐trimethylbenzene 0.22 ± 0.06. The major products identified from the reaction of O3with toluene, o‐xylene and 1,3,5‐trimethylbenzene were α‐dicarbonyls.