We report accurate ab initio studies of the static dipole polarizabilities and first hyperpolarizabilities of acenaphthene and acenaphthylene. Geometries were optimized at the HF/6-311G(3d,2p) level of theory. Dipole polarizabilities were calculated at the HF/6-311++G(3d,2p) and BLYP/6-311++G(3d,2p) levels of theory, using HF/6-311G(3d,2p) geometries. The title molecules are predicted to have very similar mean polarizabilities of 142 a.u. A comparison is made with corresponding calculations on naphthalene. Semi-empirical AM1 calculations of the dipole polarizability are also given for comparison; the normal component of the dipole polarizability tensor is poorly represented by such calculations.