Reactions of ozone with some vinyl compounds of the general structure CH₂CH–X were studied in aqueous solution. Rate constants (in brackets, unit: dm³ mol⁻¹ s⁻¹) were determined: acrylonitrile (670), vinyl acetate (1.6 × 10⁵), vinylsulfonic acid (anion, 8.3 × 10³), vinyl phenylsulfonate (ca. 200), vinyl diethylphosphonate (3.3 × 10³), vinylphosphonic acid (acid, 1 × 10⁴; mono-anion, 2.7 × 10⁴; di-anion, 1 × 10⁵), vinyl bromide (1 × 10⁴). The main pathway leads to the formation of HOOCH₂OH and HC(O)X. As measured by stopped flow with conductometric detection, the latter one may undergo rapid hydrolysis by water, e.g.HC(O)CN (3 s⁻¹). Other HC(O)X hydrolyse much slower, e.g.HC(O)PO₃(Et)₂ (7 × 10⁻³s⁻¹) and HC(O)P(OH)O₂⁻ (too slow to be measured). The OH⁻-induced hydrolyses range from ca. 5 dm³ mol⁻¹ s⁻¹ [HC(O)PO₃²⁻] to 3.8 × 10⁵ dm³ mol⁻¹ s⁻¹ [HC(O)CN]. HC(O)Br mainly decomposes rapidly (too fast for the determination of the rate) into CO and Br⁻ plus H⁺, and the competing hydrolysis is of minor importance (3.7%). The slow hydrolysis of HC(O)PO₃²⁻ at pH 10.2, where HOOCH₂OH is rapidly decomposed into CH₂O plus H₂O₂, allows an H₂O₂-induced decomposition (k = 260 dm³ mol⁻¹ s⁻¹) to take place. Formate and phosphate are the final products.