The ability to routinely acquire measurements of the ozone density profile in the mesosphere and lower thermosphere is important for use in chemical-dynamical models of the middle atmosphere. Zenith measurements of the O2(1Δg) (0,1) band emission rate in the evening twilight were acquired near the spring equinox of 1982 with a Michelson interferometer from London, Ontario, Canada. Knowledge of the change of the O2(1Δg) (0,1) band emission rate at twilight can be related directly to ozone density, since ozone destruction by sunlight is the primary source of O2(1Δg) in the atmosphere. Measurements and calculations have shown that a secondary peak in the ozone density often exists in the middle atmosphere. A model has been developed to infer the ozone profile in the middle atmosphere by simultaneously solving the time-dependent chemistry of the molecular oxygen atmospheric and atmospheric-IR bands and O(1D) during twilight. Calculations are presented which show the effect of a secondary peak in the ozone density at various heights on the O2(1Δg) (0,1) band emission rate during twilight. The model is used to demonstrate that the London measurements are consistent with an ozone profile with a secondary peak at 85--90 km. ¿ American Geophysical Union 1993