The photolyses of acetyl and ethyl isocyanate vapors have been studied at 25 °C, at pressures between about 2 and 30 Torr, using either monochromatic 2288 Å radiation or light from a medium-pressure mercury arc equipped with a Vycor filter. Molecular photodissociations were of major importance with both compounds, with formation of HNCO + CH2CO and HNCO + C2H4accounting for about 70 and 60% of the decomposition of the acetyl and ethyl esters, respectively. It is suggested that these products were formed through a 6-membered ring transition state analogous to that involved in the Norrish Type II process observed in other carbonyl compounds.Minor free-radical processes also occurred with both isocyanates, with evidence for photodissociation of both R—NCO and RN—CO bonds. There was also apparently some collisional deactivation of excited precursors of both the molecular and free-radical photodissociations.Vapor pressure data for the 2 isocyanates, and molar extinction coefficients between about 2200 and 3000 Å, are also reported.