The thermal decomposition of pyruvic acid diluted in Ar has been studied behind reflected shock waves over the temperature range 850-1000 K, with a total density range of (0.3-1.3) x 10(-5) mol/cm(3). The decomposition process was monitored by time-resolved IR emission from carbon dioxide product. The vacuum-UV absorption at 193 nm suggested the production of hydroxyethylidene from the initial decomposition. This carbene isomerizes to acetaldehyde at temperatures higher than those used under the present conditions. From the experimental results, we propose that the initial reaction occurs through a five-center transition state (TS) as CH3COCOOH --> CH3COH + CO2 with a rate constant k = 10(12.46) exp(-40.0 kcal mol(-1)/RT) s(-1). An ab initio molecular orbital calculation at the HF/6-31G**//HF/3-21G level shows that the energy of the five-center TS producing hydroxyethylidene is lower by 60 kcal/mol than that of the four-center TS leading directly to acetaldehyde. A theoretically evaluated rate constant agrees with the experimental value, in support of the proposition that the reaction path occurs through the five-center TS to produce the carbene radical. Also, it appears that the carbene subsequently isomerizes through two routes leading to acetaldehyde and/or vinyl alcohol.