The rate constants (k) for solvolysis of a range of 3-X-substituted bicyclo[1.1.1]pentyl bromides in 80% aqueous ethanol have been measured and a linear relationship found to exist between log k and the inductive/field constant sigma(I). These observations, together with the results of isotope and solvent effect studies described below, have been interpreted as evidence for the formation of 1-bicyclo[1.1.1]pentyl cations in the ionization step in preference to mechanisms in which either ring-opening is concerted with ionization to give 3-methylenecyclobutyl cations or ionization is accompanied by ring-contraction to give bicyclo[1.1.0]butyl-1-carbinyl cations. The latter mechanisms would have been expected to show the effects of strong pi-donor substituent participation, a phenomenon which was not observed. A comparison of the rates of solvolysis of the parent bromide and its 3-deuterated analog is characterized by a large deuterium-isotope effect (k(H)/k(D) = 1.30) which, combined with the observation that 1-bromobicyclo[1.1.1]pentane solvolyses with enhanced rates in solvents which are strong hydrogen bonding accepters, lends support to the view that the 1-bicyclo[1.1.1]pentyl cation derives substantial stabilization from sigma-hyperconjugative interactions.