Recent state-selected photodissociation experiments on isocyanic acid, HNCO, have provided a wealth of data on its photochemistry and dissociation dynamics. The excited state potential energy surface on which the dissociation occurs is central to these observations but is relatively uncharacterized. We construct a two-dimensional analytical model for the excited state potential that is consistent with experimental observations, including the ultraviolet absorption spectrum and the dynamics of the C-N and N-H bond dissociations. We then test this surface by running classical trajectories on it, using Morse oscillator vibrational wave functions from the ground electronic state to determine the probability distributions of initial conditions. The trajectory calculation reproduces the experimentally observed Variation in the photochemical branching with photolysis wavelength. It also reproduces the bond selectivity in the photodissociation of HNCO molecules containing three quanta of N-H stretching excitation (3 v(1)) that we observed experimentally. Although the model for the surface is very simple and includes only two degrees of freedom, it captures the essential features that determine the photochemical branching in a direct dissociation.