The heats of formation of ketene, methylketene, and dimethylketene have been measured by energy-selected electron-induced dissociation of phenyl acetate, propanoate, and isobutyrate, respectively. The values are Delta(f)H degrees[CH2=C=O] -54 +/- 5 kJ mol(-1), Delta(f)H degrees[CH3CH=C=O] = -95 +/- 5 kJ mol(-1), and Delta(f)H degrees-[(CH3)(2)C=C=O] = -137 +/- 5 kT mol(-1). For ketene, the new result was in excellent agreement with literature data and recent high-level ab initio calculations. The values for the methyl-substituted analogues differed significantly from those obtained by the recent calculations. The heats of formation of ionized methyl-and dimethylketene were also measured to be 765 and 683 +/- 5 kT mol(-1), respectively. These experimental results show that the effect of methyl substitution in neutral ketene decreases the heat of formation by ca. 40 kT mol(-1) per methyl group, which is in keeping with the effect of methyl substitution in structurally related systems.