Iron(II) sulfate converts t-butyl oxaziridines, derived either from 5-t-butyl-2H-pyrrole 1-oxides or from 2-t-butyl-3-alkylidene- or 3-arylmethylene-1-pyrroline 1-oxides by photorearrangement, into 3- pyrrolin-2-ones, 3-alkylidenepyrrolidin-2-ones or 3-arylmethylenepyrrolidin-2-ones respectively.