Rate constants have been determined for proton transfer reactions of the type [MH](+) + B reversible arrow M + [BH](+), where M is ketene, methylketene, and formaldimine and B is a reference base. A quantitative relationship between the rate constant and the free energy (or enthalpy) of the reaction allows the determination of the gas-phase basicity, GB, (or proton affinity, PA) of M. This thermokinetic method gives results comparable to that obtained from equilibrium constant measurements. The values derived for ketene, methylketene, and formaldimine follow : GB(ketene) = 788 +/- 3 kJ/mol, PA(ketene) = 817 +/- 3 kJ/mol; GB(methylketene) = 809 +/- 3 kJ/mol, PA(methylketene)= 842 +/- 3 kJ/mol; GB(formaldimine) = 830 +/- 3 kJ/mol and PA(formaldimine) = 860 +/- 5 kJ/mol. Heats of formation of methylketene and of formaldimine that may be deduced from a combination of these results with literature data are as follows : Delta H-f degrees(300) (CH3CHCO) = -97 +/- 5 kJ/mol and Delta H-f degrees(300) (CH2=NH) = 75 +/- 5 kJ/mol.