cis-1,4-Diphenyl-3-phthalimidoazetidin-2-one was epimerised completely to the corresponding trans-β-lactam in presence of 1,5-diazabicyclo[4,3,0]non-5-ene in benzene solution at 100° in about 22 h but the trans-isomer was unchanged on similar treatment: an equilibrium mixture containing 30% of the cis-isomer was obtained when either the cis- or the trans-3-bromo-1,4-diphenylazetidin-2-one was heated on a steam bath with the same base in benzene solution or treated with a solution of the base in Me2SO at room temperature.