Ab initio Hartree-Fock (HF) and density functional theory (DFT) calculations of the electric dipole-magnetic dipole polarizability beta(alpha beta) are reported for the chiral molecules methyloxirane (1) and trans-dimethylthiirane (2) in the static limit. Values of beta = 1/3Tr[beta(alpha beta)] obtained thence are used to predict the specific optical rotations [alpha](D) of 1 and 2. Gauge-including atomic orbitals (GIAOs) are used to ensure origin independence of beta and [alpha](D). beta and [alpha](D) values converge slowly to the complete basis set limit. Basis sets including diffuse functions are required to achieve reliable results. HF and DFT values of beta(alpha beta) differ generally by 10-30%. Calculated [alpha](D) values for 1 and 2 obtained using large basis sets agree well with experimental values in sign and magnitude. The deviations can be attributed in part to the neglect of the frequency dependence of beta and of solvent effects.