Temperature programmed desorption (TPD) and 365 nm photolysis of ClNO adsorbed on MgO(100) at 90 K were investigated under ultrahigh vacuum conditions. The crystal was treated in a way that largely eliminated oxygen vacancies and yielded a relatively smooth surface. Angularly resolved time-of-flight (TOF) mass spectra and state-selective resonance-enhanced multiphoton ionization (REMPI) spectra of NO photoproducts were obtained. The TPD data indicate that ClNO desorbs at surface temperatures above 160 K for exposures (Theta) below 0.2 Langmuirs (L), while for higher values of Theta the main desorption peak is near 120 K. The higher temperature feature, which saturates at Theta similar to 0.3 L, is probably associated with binding to defect sites. Thermal desorption is believed to be molecular at ail coverages. irradiation at 365 nm for 0.1 less than or equal to Theta less than or equal to 5.0 L yields products having low average translational energies and broad translational energy distributions. NO fragment REMPI spectre were recorded at Theta greater than or equal to 0.7 L. The rotational distributions could be fit with a temperature of 110+/-10 K, i.e., comparable to that of the substrate. These results differ from those obtained in the photodissociation of gas-phase ClNO, where the NO fragment has high translational and rotational energies. However, the present results are similar to those obtained on rougher MgO(100) surfaces. Possible mechanisms are discussed.