The 4--photocyclization of a range of 1,2dihydropyridazines is described. The bicyclic 1,2-diazetidine products were generally obtained in high yields, and the process was successfully scaled up to multigram scale. The key bicyclic 1,2-diazetidines are versatile synthetic intermediates and were easily converted into a range of novel derivatives, including functionalized 1,2diazetidines, cyclobutenes, cyclobutanes, and dienes.