Molecular beam studies of the temperature dependence of n-butane fragmentation patterns were carried out under several conditions. Under effusive-flow molecular-beam formation, the temperature dependence of fragmentation to several ions from n-butane agrees well with conventional studies. Under free-jet expansion molecular beam formation, the fragmentation of n-butane to several ions indicates substantial vibrational relaxation of the neutral molecule. Dilution of n-butane with argon at room temperature gives rise to parent/fragment ion ratios that suggest that the dimer or higher clusters are contributing substantially to ion peaks normally attributed to the monomer. The unusual ions (C4H10)n+ are readily generated. Implications of the large observed vibrational cooling and its method of detection are discussed.