It is shown by pulse radiolysis that in N2O-saturated methylcyclohexane (MCH) the solvent radical cation MCH+ is formed, but in argon-saturated MCH, the olefinic fragment cation methylcyclohexene(+) (MCHexene(+)) is obtained. From simulations of the geminate ion kinetics with the t(-0.6) semiempirical rate law, it is concluded that both cations must have a common precursor: some excited state of MCH+, called M+*. This precursor either fragments to form MCHexene(+) or is quenched, e.g. by N2O, to form MCH+, or relaxes in the NIGH environment. The corresponding rate constants at 143 K are k(frag) = (2.5 +/- 0.5) x 10(6) s(-1), k(2)(M+* + N2O) (3.1 +/-0.5) x 10(7) M-1 s(-1) and k(0) = (3.2 +/- 1.6) x 10(5) s(-1). The mobility of M+* was assumed to correspond to that of the fast MCH+, i.e. DM+* approximate to DMCH+ = (1.8 +/- 0.2) x 10(-6) cm(2) S-1. The mobility of the solvated electron, D-e-(sch), was determined to be (1.6 +/- 0.2) x 10(-6) cm(2) s(-1). From a Lorentzian line shape analysis of the free ion intercept spectra, the individual yields for both systems were derived. The optimal line parameters are the following: for MCH+ lambda(max) = 570 nm, hwhm (half-width at half-maximum) = 19000 cm(-1); for MCHexene(+) lambda(max) = 450 nm, hwhm = 5500 cm(-1); and for e(solv)(-) lambda(max) = 1900 nm, hwhm = 4500 cm(-1). Using the known free ion yield (G(fi) = 0.06 +/- 0.015 (100 eV)(-1)) the absorption coefficients are calculated to be: epsilon(MCHexene(+))(450nm) = 2260 +/- 200 M-1 cm(-1), epsilon(MCH+)(570nm) = 1910 +/- 150 M-1 cm(-1), epsilon(e(solv)(-))(700nm) = 1100 +/- 50 M-1 cm(-1), and epsilon(M+*)(600nm) = 700 +/- 50 M-1 cm(-1). The error limits given are due to the line shape analysis only. There is an additional systematic effect due to the error-limit for the G(h)-value (+/-25%):all epsilon-values become higher if the G(fi)-value should be smaller.