Methylisothiocyanate (CH(3)NCS) was photolyzed at 193 and 248 nm, and the resulting time-resolved infrared emission was observed. Similar experiments were performed on methylthiocyanate (CH(3)SCN) photolyzed at 193 nm. Previous work suggested that these isomers undergo excited-state isomerization prior to dissociation, but other experiments have contradicted this claim. In the infrared emission experiments, we observed the same products from both starting materials, supporting the theory of excited-state isomerization prior to dissociation. Methylisothiocyanate is the active ingredient in a widely used pesticide and has been observed to form highly toxic methyl isocyanate (CH(3)NCO) under environmental conditions. The mechanism for this formation has been unclear, but must involve some oxygen-containing species. At 248 nm, methylisothiocyanate was photolyzed alone and with three atmospheric oxidizers: O(2), NO, and NO(2). No chemical reaction was observed with O(2), whereas secondary reactions were observed with NO and NO(2). When methylisothiocyanate was photolyzed with NO(2), methyl isocyanate (CH(3)NCO) was observed, suggesting a likely environmental mechanism for methyl isocyanate formation.