Three-dimensional potential energy surfaces for the (X) over tilde(1)A', (A) over tilde(1)A '', and (a) over tilde(3)A '' states of HNO have been calculated at the multireference configuration interaction (MRCI) level of ab initio theory. Energy points are calculated at 1728 molecular configurations, predominantly sampling the HNO well and the H+NO product channel regions. Energies between grid points are obtained by interpolation with a three-dimensional cubic spline. The well depths are 2.14, 1.27, and 0.38 eV for (X) over tilde(1)A', (a) over tilde(3)A '', and (A) over tilde(1)A '', respectively. Saddle points to inversion, isomerization, and dissociation are reported and their importance discussed to relevant processes. The HNO((X) over tilde(1)A') potential energy surface is purely attractive along its minimum energy path to ground-state products, whereas the (a) over tilde(3)A '' and (A) over tilde(1)A '' states have barriers of 0.21 and 0.50 eV, respectively. Vibrational term values and rotational constants for HNO and DNO are reported for the fundamental vibrations for all three electronic states. Where comparison with experimental data is possible, the agreement is satisfactory. The three potential energy surfaces are appropriate to study the predissociation dynamics in the (A) over tilde(1)A '' state and the unimolecular dissociation in the (X) over tilde(1)A' state. (C) 1997 American Institute of Physics. [S0021-9606(97)01141-0].