The kinetics and mechanism for the thermal decomposition of diketene have been studied in the temperature range 510-603 K using highly diluted mixtures with Ar as a diluent. The concentrations of diketene, ketene, and CO2 were measured by FTIR spectrometry using calibrated standard mixtures. Two reaction channels were identified. The rate constants for the formation of ketene (k1) and CO2 (k2) have been determined and compared with the values predicted by the Rice-Ramsperger-Kassel-Marcus (RRKM) theory for the branching reaction. The first-order rate constants, k 1 (s-1)= 1015.74±0.72 exp(-49.29 (kcal mol-1) (±1.84)/RT) and k2 (s-1)= 10 14.65±0.87 exp(-49.01 (kcal mol-1) (±2.22)/RT); the bulk of experimental data agree well with predicted results. The heats of formation of ketene, diketene, cyciobuta-1,3dione, and cyclobuta-1,2-dione at 298 K computed from the G2M scheme are -11.1, -45.3, -43,6, and -40,3 kcal mol-1, respectively.