The classical-plus-tunneling method of Waite and Miller [J. Chem. Phys. 73, 3713 (1980)] has been used to study tunneling effects in the cis-trans isomerization of HONO. The, calculated classical-plus-tunneling rate coefficients are two and three times larger than the corresponding purely classical values for trans-->cis and cis-->trans isomerizations, respectively. The tunneling rate is governed by the rate of energy flow into the tunneling coordinate, the effective mass, and the tunneling frequency. An approximation to the actual torsional potential used in the classical dynamics calculations was developed to give an analytical solution for the Jeffreys-Wentzel-Kramers-Brillouin (JWKB) barrier-penetration integral. The approximation yields accurate results (compared to those obtained by solving the JWKB integral numerically using the true torsional potential) and a considerable reduction in computational expense.