Energy partitioning in terms of symmetry components shows that sigma-bonding changes contribute importantly to the energetics of the (similar to 700 cm(-1)) methyl torsional barrier in propene. This a-conjugation effect, larger in propene than in acetaldehyde, has its primary origin in the partial unmaking of the bond between the methyl carbon and adjacent carbon atoms. The role of strain in generating torsional barriers is discussed. Rigid rotation leaves both molecules in a strained metastable conformation with the barrier primarily originating from ir contributions to a repulsive nuclear virial. But overall energetics and hence barrier heights in the relaxed molecule are controlled by a interactions. Extension to the propene(+2) pi cation provides insight into competition between a and pi interactions forming the barrier. In neutral propene dominant pi-electron repulsion decreases the barrier, in the cation dominant sigma-electron repulsion increases the barrier {predicted [MP2/6-311G(3df,2p)] height, 1241 cm(-1)}.