The contributions of individual substituents to properties of fluorochloroethanes are highly nonadditive. The nonadditivies can be accurately modeled with sums of terms describing geminal, gauche vicinal, and anti vicinal interactions between substituents. Such an approach, which requires 12 parameters per property, produces the total and zero-point energies and the lengths of the C-C bond with rms errors of 0.8 and 0.04 kcal/mol and 0.002 Angstrom, respectively. The inclusion of all three types of interactions is prerequisite to obtaining reasonably accurate estimates of molecular properties, although in the cases of the total and zero-point energies the geminal contributions strongly dominate. The present methodology is very general, making it possible to analyze and predict substituent effects in a systematic manner by summing transferable increments ascribed to individual substituents, substituent pairs, triples, etc., until the desired accuracy is attained. The substituent effects in the molecules under study cannot be attributed solely to the phenomenon of electronegativity equalization.