The effect of fluorine substitution on the energies of small ring compounds has been examined via ab initio calculations at the MP2/6-31G*, MP2/6-311+G*, and B3LYP/6-311+G* theoretical levels, along with correction for differences in zero-point energies. The introduction of a fluorine into a cyclopropane ring leads to destabilization, which results from the higher s character in the orbitals forming the bonds to a substituent. On the other hand, this is not seen with cyclopropene. The effect of 3-substituents on cyclopropene was examined by studying cyclopropenone, cyclopropenethione, methylenecyclopropene, and 3,3-difluorocyclopropene. The stabilization was largest and about equal with the =O and =S substituents. Smaller effects were observed with =CH2 and F-2 substituents. The nature of the effects was studied making use of electron density difference maps. The structures and energies of the series of C4H4 and C4F4 derivatives were studied at the above theoretical levels. In most cases, fluorine substitution led to stabilization with respect to but-3-en-1-yne, but with tetrahedrane there was considerable destabilization. Fluorine substitution leads to destabilization of cyclobutyne with respect to cyclobutene.