The ionization of l-bicyclo[1.1.1]pentyl halides was shown to initially form the 1,3-bridged bicycle[1.1.1]pentyl-l cation. It appears to be a transition state that leads to the bicyclo[1.1.0]butyl-1-carbinyl cation which can be trapped with azide ion and can be directly observed by NMR in SO2CIF. Although the major products of solvolysis of the halides are 3-methylenecyclobutyl derivatives, the corresponding cation was calculated to have a significantly higher energy than the bicyclobutylcarbinyl ion. Therefore, the products are probably formed by an attack of the nucleophile on the latter ion, accompanied by bond migration. The bridgehead iodide reacts under solvolytic conditions with azide ion to form bicyclo[1.1.0]butyl-l azide as a product. It also reacts with potassium hydroxide to give [1.1.1]propellane, and the same reaction occurs on dissolving in acetonitrile or pyridine. The reaction of 1,3-diiodobicyclo[1.1.1]pentane with ethoxide ion also was found to give [1.1.1]propellane via a nucleophilic attack on one of the iodines. The propellane reacts with methyl hypoiodite to give 3-iodobicyclo[1.1.1]pentyl-l cation, which can react with methanol to give 3-methoxybicyclo[1.1.1]pentyl-l iodide and with azide ion to give 3-iodobicyclo[1.1.1]pentyl-1 azide. These data provide evidence for a discrete 3-iodobicyclo[1.1.1]pentyl-l cation intermediate. The effect of substituents on the rate of solvolysis of bicyclo[1.1.1]pentyl-l iodide was studied. With 3-aryl substituents, a value of rho = -1.7 was found, which is similar to that observed in the solvolysis of 3-arylcyclobutyl tosylates (rho = -1.6). The 3-substituted bicyclopentyl halides usually form the corresponding 3-methylenecyclobutyl cations rather than bicyclo[1.1.0]butyl-1-carbinyl ions, because most substituents will help stabilize the former type of ion.