Infrared spectroscopic analysis of the products showed that flash vacuum pyrolyses (FVP) of dimethyl tert-butylmalonate ( 1b) and methyl tert-butyl(pivaloyl)acetate ( 1d) at ca. 550 °C afforded the corresponding tert-butyl(carbomethoxy)ketene ( 4b) and tert-butyl(pivaloyl)ketene ( 4d), respectively, with loss of methanol, together with unreacted 1b and 1d (Ar matrix, 12 K or neat at 77 K; 10 −5 mbar). Monitoring by IR spectroscopy showed that 4b reacted with methanol at ca. −50 °C to give 1b. Ketene 4d does not react with methanol at room temperature, but afforded ester 1d on refluxing for 8 h. FVP of 1b and 1d at temperatures above 650 °C gave the α-oxoketenes 4b and 4d, respectively, unsubstituted dimethyl malonate ( 1a) and methyl 4,4-dimethyl-3-oxopentanoate ( 1c), respectively, due to retro-ene reactions with elimination of isobutene, as well as pyrolysis products derived from 1a and 1c, respectively. FVP of α-unsubstituted β-ketoesters 1a and 1c at ca.500 °C (10 −5 mbar) with argon matrix isolation of the products at 12 K afforded the ketenes 4a and 4c as mixtures of s- Z and s- E conformers together with mixtures of unreacted keto ( 1a.c) and enol forms ( 2a.c). On warming to temperatures between −90 and −50 °C, back-reaction of ketenes 4a,c with methanol resulted in the re-generation of enols 2a,c, without increasing the amounts of the keto forms 1a,c.