Using the potential energy surface and molecular properties obtained at the accurate composite W1U method, time-resolved temperature- and pressure-dependent behaviors of the thermal decomposition of beta-propiolactone and beta-propiolactam (for 500-2000 K & 0.001-760 Torr) were charaterized using the integrated deterministic/stochastic model within the master equation/Rice-Ramsperger-Kassel-Marcus (ME/RRKM) framework. The integrated model helps to confidently explain and predict the different kinetic behaviors of the two similar-ring systems, which is consistent with experimental data, namely, beta-propiolactone only decomposes to C2H4 + CO2 while beta-propiolactam can form both C2H4 + HNCO (main product) and CH2CO + CH2NH (minor product). (C) 2017 Elsevier B.V. All rights reserved.