It is shown by pulse radiolysis that the transient absorption in N2O-saturated or CO2-saturated methylcyclohexane (MCH), peaking at about 570 nm, is due to the solvent radical cation MCH(+). From simulations with the semiempirical t(-0.6) rate law, it is concluded that MCH(+) is of very high mobility : at 133 K (supercooled liquid) D-exp = (1.06 +/- 0.2) x 10(-6) cm(2) s(-1), which is ca. 400 times faster than expected from diffusion. At room temperature MCH(+) is about 11 times faster than diffusion, and this is in perfect agreement with the conductivity of MCH(+), as measured by Warman et al. For the high mobility of MCH(+) an activation energy of 8.9 +/- 0.3 kJ/mol is found. The rate constant for scavenging MCH(+) with norbornadiene (NBD) is k(2)(133 K) = (1.8 +/- 0.5) x 10(8) M(-1) s(-1). This is again 80 times faster than the diffusional rate constant. From the intercepts of the semiempirical t(-0.6) linearity plots, the free ion spectra were derived. The free ion absorbance at 133 K turns out to be 2.0 times smaller than that at room temperature. The free ion yield at low temperatures therefore was derived to be G(fi)(127-153 K) = 0.06 +/- 0.015 (100 eV)(-1). From the free ion intercept at room temperature the absorption coefficient epsilon was determined : epsilon(570nm)(MCH(+)) greater than or equal to 2300 M(-1) cm(-1). Without the electron scavengers N2O or CO2 the olefinic cation methylcyclohexene(+) is found to replace MCH(+). This indicates that some excited species, usually quenched by N2O or CO2, is the precursor of the high-mobility radical cation MCH(+).