The mass spectrometric fragmentation of methylcyclohexane has been studied with high mass resolution. The use of 25 isotopomers, labeled with 13 C and/or D, allowed the elucidation of different fragmentation mechanisms, but demonstrated also the limits of the method. Except for the loss of methyl radical, all neutral fragments are lost by several pathways, some of them preceded by a randomization of the hydrogen atoms. The general rule confirms that saturated hydrocarbons suffer complex fragmentation because the charge is not necessarily well located. Having seen the difference betwen the results of the metastable (a few μs) and the ion cyclotron resonance (10 ps‐10 ms) time window fragmentation, it is probable that these difference mechanisms follow different time patterns. Collision activation allowed the study of the fragmentation pattern as a function of the relative collision energy.