The T-1(n,pi*) <-- S-0 laser-induced phosphorescence excitation spectrum of biacetyl has been recorded with a rotating slit nozzle excitation apparatus. The O-0(0) system origin was observed for the first time as a weak band at 19 529 cm(-1). Built on this band were the activities of the three large amplitude modes: upsilon(16)(a(u)) acetyl torsion, upsilon(15)(a(u)) gearing methyl torsion, and upsilon(21)(b(g)), the antigearing methyl torsion. The potential surface for methyl internal rotation extracted from fitting the observed levels yielded a barrier to methyl torsion of 359.6 cm(-1).