A photoionization mass spectrometric study of HNCO yields the ionization potential (I.P.) (HNCO)=11.595+/-0.005 eV and the appearance potential (A.P.) (NCO+/HNCO)less than or equal to 16.53(2)+/-0.01(1) eV at 0 K. A similar study of NCO (generated by F+HNCO) gives I.P. (NCO)=11.759+/-0.006 eV. These observations lead to D-0 (H-NCO)less than or equal to 110.1+/-0.3 kcal/mol. Additional analysis enables one to infer 28.4+/-0.5 kcal/mol less than or equal to Delta H-f0(0) (NCO)less than or equal to 32.8+/-0.7 kcal/mol. The implication of these results for kinetic modeling of the processes for reduction of NOx is discussed.