Computational studies of carbenium ions relevant to sterol biosynthesis via lanosterol synthase were undertaken to determine fundamental energetics underlying cyclization steps. Ab initio B3LYP/6-31G*//B3LYP/6-31G* calculations were performed for the addition of 2-methyl-2-propyl cation and 2-methylpropene to represent a tertiary cation --> tertiary cation cyclization. Solvent effects were included by Monte Carlo (MC) simulations in methylene chloride, methanol, and THF. The picture that emerges for a cation-olefin cyclization is one of barrierless collapse at short distance, while desolvation and conformational barriers are expected for initial separations beyond ca. 5 Angstrom. Thus, the cyclization that forms the sterol B ring likely proceeds in barrierless concert with A-ring formation in the preorganized environment of a cyclase enzyme. However, C-ring formation appears to involve a tertiary secondary cation rearrangement. This was modeled by ab initio and MC calculations for the interconversion of the C-11 cations, 9 and 10. The 12 kcal/mol higher energy for the secondary ion is only reduced to ca. 10 kcal/mol by solvation. Though this is consistent with initial formation of the tertiary ion, force-field calculations on the full protosteryl cations show that the equilibrium between the isomeric ions can be readily shifted by selective placement of nucleophilic groups from the protein backbone or side chains including the indole ring of tryptophans.