Tetraisopropylmethane (1) exists in solution as a mixture of two types of conformers (D-2d and S-4 time-averaged symmetry) in the ratio 93:7 at - 110 degreesC, interconverting with a barrier of 9.7 kcal mol(-1). Molecular mechanics calculations and the multiplicity of NMR signals at low temperature allow the assignment of these conformations. The only conformation populated in tetracyclopropylmethane (2) is the same type as the minor conformation (S-4 time-averaged symmetry) populated in 1. 13 C NMR spectra at about -180 degreesC show that degenerate versions of this conformation interconverting with a barrier of 4.5 kcal mol-1. Molecular mechanics calculations that characterize the six possible conformational types for these molecules, and the most important interconversion pathways, are reported. Calculated and experimental barriers match satisfactorily well.