In aqueous solution, ozone reacts with 4-methoxycinnamate, cinnamate and 4-nitrocinnamate with rate constants of 6.8 × 105, 3.8 × 105 and 1.2 × 105 dm3 mol−1 s−1, respectively. The corresponding acids react somewhat more slowly. In product studies, material balance with respect to ozone consumption has been obtained. In the case of cinnamic acid and its 4-methoxy derivative, glyoxylic acid, H2O2 and the corresponding benzaldehydes are formed. In contrast, 4-nitrocinnamic acid affords, besides a full yield of 4-nitrobenzaldehyde, 70% glyoxylic acid and 30% formic acid, while the H2O2 yield is also reduced to 70%. These results can be explained if in the latter case the Criegee intermediate decomposes not only into glyoxylic acid and 1-hydroperoxy-1-phenylmethanol (which rapidly releases H2O2 yielding 4-nitrobenzaldehyde, k > 0.5 s−1), but also, to an extent of 30%, into 4-nitrobenzaldehyde and 2-hydroperoxy-2-hydroxyacetic acid. This product has been produced independently by ozonating fumaric acid and has been shown to decompose rapidly (k > 1 s−1) into formic acid and CO2. The reaction of H2O2 with the free carbonyl form of glyoxylic acid, present at 1.8% in equilibrium with its hydrate, is comparatively slow (k = 0.3 dm3 mol−1 s−1, based on total glyoxylic acid concentration). The reaction of the stronger nucleophile HO2−, present in basic solutions, is considerably faster (k = 1700 dm3 mol−1 s−1). Thus, in the cinnamate system, the substituents markedly influence the fate of the Criegee intermediate, although they have only a small influence on the rate of the ozone reaction.