The unimolecular decomposition of 1,3,5‐trioxane into three formaldehyde molecules has been studied thermally at eight temperatures between 523 and 603 K using mixtures highly diluted with Ar. Under these conditions, the only decomposition product detected by means of FTIR analysis was formaldehyde. At 588 K, the effect of total pressure was examined between 25 and 800 torr; a noticeable decline in the first‐order rate constant was observed only at pressures below 500 torr. A least‐squares analysis of the measured high‐pressure, first‐order rate constants leads to (Formula Presented.) This result differs significantly from earlier data on the reaction, but it compares closely with the theoretical value: (Formula Presented.) computed with the transition‐state theory using the molecular and TST parameters predicted by the BAC–MP4 method.