The heat of formation of the methylcarbyne radical (CH3C) is calculated using various ab initio approaches. The most accurate value, at the CCSD(T)/6-311G(3df,3pd)//MP2/6-311G(3df,3pd) level of theory, is derived as Delta H-f(298)[CH3C((X) over tilde(2)A ")] = 122 +/- 1 kcal mol(-1); in addition Delta H-f(298)[CH3C((a) over tilde(4)A(2))] is calculated as 152 +/- 2 kcal mol(-1). The (a) over tilde(4)A(2)-(X) over tilde(2)A " excitation energy is derived as 1.3 +/- 0.1 eV [29 +/- 2 kcal mol(-1)]. For both electronic states, the equilibrium geometry and harmonic vibrational frequencies are calculated. The use of the Gaussian-2 (G2) theoretical model to calculate the maximum electron kinetic energy from chemiionization reactions is explored. In particular, the CH3C + O --> CH3CO+ + e(-) and CH + O --> HCO+ + e(-) chemiionization reactions at 298 K are considered for the doublet and quartet states of the hydrocarbon radical. These calculations lead to a maximum electron kinetic energy of 1.04 and 2.46 eV for the former reaction, and 0.22 and 1.05 eV for the latter reaction, where the first number in each case refers to the ground doublet state of the reacting radical and the second refers to the lowest quartet. It is concluded that the G2 method is adequate for determining the thermodynamics of chemiionization reactions involving species in their ground electronic states; however, a higher level of theory is required for excited states.