We have investigated under which physical conditions (kinetic temperature, H2 density, CO column density) the CO molecule shows suprathermal excitation and population inversion. The computations are based on a model in which the excitation is basically due to collisions with H2 molecules. The collision cross-sections were taken from Green and Thaddeus (1976). The radiative transport in the molecular lines is treated in an on-the-spot approximation (see e.g. Kegel 1979)

J = S(1 - e-τ) + Ibge-τ

in order to keep the computing time low enough to permit the investigation of a wide range of parameters. Our approximation of the radiative transfer is mathematically equivalent to a formalism involving an escape probability β = e-τ. This escape probability arises from the fact that the radiative transfer in the line wings is neglected.