The homogeneous thermal decomposition of acetic acid has been studied between 530 and 762° in a flow system. Below 600° dehydration is second-order: k= 10^9·52 exp(–39,300/RT) l. mole^–1sec.^–1, but the order falls and the Arrhenius plot curves upwards above this temperature. Above 700°, the process is first-order: k= 10^12·45 exp(–64,890/RT) sec.^–1. Decarboxylation, however, is first-order throughout the range: k= 10^13·59 exp(–69,800/RT) sec.^–1. All processes are molecular.