An updated kinetic model for the calculation of daytime steady state O2(a1Δg) concentrations in the mesosphere and lower thermosphere is presented. In addition to the usual source from ozone photolysis in the Hartley band, energy transfer from O(1D) produced by photolysis of molecular oxygen in the Lyman alpha and Schumann-Runge continuum spectral intervals is shown to be a significant source of O2(a1Δg) as well. Numerous changes and additions to the basic O2(a1Δg) kinetic model have been included. These new sources and kinetic parameters imply that lower mesospheric ozone concentrations as inferred by near-infrared emission techniques are to be decreased by as much as 10% while lower thermospheric ozone concentrations are to be increased by as much as 20%.