The thermal decomposition of 3,4-dihydro-2H-pyran (DHP, C~H B O ) has been investigated by two methods: in shock waves with the laser-schlieren technique using mixtures of 5 and 10% DHP in krypton over 900-1500 K, 110-560 torr; in a flow tube having a reaction pressure 0.5 torr above atmospheric, using the decomposition of allylethyl ether as an internal standard, and covering 663-773 K. The retro-DielsAlder dissociation to the stable acrolein and ethylene is the dominant channel for all conditions. Precise rate constants (rms deviation of 10%) were obtained for this process over the indicated temperature ranges. Unimolecular falloff is evident in the shock-tube results, and RRKM calculations also predict a slight falloff a t the lower temperatures. These RRKM calculations use a routine vibration model transition state and agree closely with the high-temperature data when (AE)down is a fured 400 cm-'. Arrhenius expressions for k, derived from the two measurements are in close accord and also consistent with most previous studies of this reaction. 0 1995 John Wiley & Sons, Inc.