2-Aryl-2-methoxy-5,5-dimethyl-Δ3-1,3,4-oxadiazolines (4) and 5-aryl-2-methoxy-2,5-dimethyl-Δ3-1,3,4-oxadiazolines (5) were synthesized. Compounds 4 decompose in solution with first order kinetics. Rate constants are correlated with Hammett substituent constants (σ−) with ρ(49.2 °C) = 0.74 and 0.89 for CCl4, and CD3OD, respectively. The final products from 4 indicate that thermolysis involves the cleavage of both C—N bonds, to form N2 and, initially, a carbonyl ylide. Compounds 5, which were obtained as mixtures of cis/trans isomers containing several impurities, and which therefore gave poorer kinetic data, decomposed in CDCl3 solution with [Formula: see text] Carbonyl ylide intermediates, similar to those from the closelyrelated compounds 4, were assumed on the basis of analogy and on the basis of partial identification of products. The effects of para substituents in the aryl groups of 4 and 5 show that the transition states have greater electron density at C-2 of 4 and at C-5 of 5 than do the starting materials. In spite of the increase in electron density at C-2 of 4, the transition state must be less polar, overall, than the ground state because rate constants for thermolysis of 4 in methanol are smaller than those for CCl4, solvent. A plausible explanation for the substituent effects and the solvent effects is that the loss of N2 is concerted, with a transition state resembling more closely a spin paired 1,3-diradical than a 1,3-dipole. Alternative stepwise mechanisms, in which C2—N3 bond scission of 4 and C5—N4 bond scission of 5 are the rate-determining steps, leading to 1,5-diradical intermediates, can not be excluded on the basis of the evidence.