Abstract: It has been documented that 5-methylene-Meldrum's acid derivatives (1, 12) and their enols (2, 13) can undergo fragmentation to malonic anhydrides (4, 19), carboxyketenes (3, 16) and methyleneketene (5, 21, 35), as well as cyclization to pyrrole-3-one and thiophene-3-one derivatives 11a,b (but not furan-3-ones 11c) under the conditions of flash vacuum thermolysis (FVT). Here we report theoretical calculations at the B3LYP/6-311 + G(2d, p) and G3X(MP2) levels of theory, which allow a rationalization of these observations. The calculated activation barriers for these reactions are all of the order of 37-40 kcal mol-1. Hydroxyacetylenes (alkynols) 7 are sometimes observed in FVT reactions of Meldrum's acid derivatives. Their formation is now explained as an FVT reaction of the carboxyketenes (e.g. 3 → 7 and 32 → 34) with a calculated activation barrier of ca. 39 kcal mol-1. The cyclization of alkylamino- and alkylthio-substituted methyleneketenes 8a,b to pyrrolone and thiophenone derivatives 11a,b is found to be energetically very feasible under FVT conditions, and even in some cases in solution, with activation barriers of 33-39 kcal mol-1. This cyclization takes place via the fleeting ylidic ketene intermediates 9a,b, 25, and 37a,b, which exist in very shallow energy minima. Alkoxy-substituted methyleneketenes 8c do not cyclize in this manner due to the rather high, but in principle not impossible, activation barriers for the initial 1,4-H shifts to the ylidic ketenes 9c (ca. 47 kcal mol-1). This journal is © The Royal Society of Chemistry.