Regions of the S-1 potential energy surface of HNCO relevant to N-H and C-N bond photodissociation have been investigated with nb initio calculations. Geometries of minima and transition states on S-1, as well as those of the product photofragments and the HNCO ground state have been optimized with the CASSCF method, and their energies calculated with MRSDCI and CASPT2 methods. Deep planar trans and cis minima exist on the S-1 surface, and are connected by transition states for isomerization. The S-0-->S-1, electronic transition is brighter for trims configurations than for cis, and the initial excitation and dynamics are most likely to proceed through trans configurations. The N-H fission on S-1 has a substantial barrier; it occurs more easily through the planar cis transition state, which is about: 20 kcal/mol above the dissociation threshold, than through the trans transition state. The C-N fission on S-1 can take place both via the planar trans and the planar cis transition state with a low barrier over the dissociation threshold; the reverse barrier is estimated to be a few kcal/mol.