The present paper is devoted to a full quantum mechanical study of the cis→trans isomerization of HONO. In contrast to our previous study Richter et al., J. Chem. Phys. 120, 6072 2004 , the dynamics is now performed in the presence of an external time-dependent field in order to be closer to experimental conditions. A six-dimensional dipole surface is computed. Using a previously developed potential energy surface Richter et al., J. Chem. Phys. 120, 1306 2004 , all eigenstates up to 4000 cm−1 are calculated. We simulate the dynamics during and after excitation by an electromagnetic pulse whose parameters are chosen to efficiently trigger the isomerization. Our investigations show that there is a selective isomerization pathway.