The relative quantum yield of O2(a1Δg) produced by the photolysis of ozone has been measured over the wavelength range 270 nm≤&lgr;≤329 nm. The O2(a1Δg) fragment was detected by [2+1> resonance enhanced multiple photon ionisation (REMPI). The quantum yield of O2(a1Δg) is compared to that recommended for O(1D) and is found to follow it closely except at the longer wavelengths where O2(a1Δg) is still produced with a yield of 10% or more. ¿ American Geophysical Union 1993