The synthesis, separation, and structural assignments of some cis- and trans-4-substituted derivatives of 2-phenyl-1,3-dioxolan are described. Some of the structural assignments were based on n.m.r. correlations; the general utility of such methods is discussed. trans-4-Methoxycarbonyl-2-methyl-1,3-dioxolan is more stable than the cis-isomer, unlike 2,4-dimethyl-1,3-dioxolan, the cis-isomer of which is preferred.