Abstract Transient grating spectroscopy is used to observe slowly forming thermal gratings in a mixture (non-combusting CO/air at ⩽0.7 MPa) of two gases that are individually transparent. Thermalization in the mixture implies a photochemical process at 532 nm that populates the metastable singlet level 1 Δ g in O 2 through collision-assisted absorption in O 2 pairs. The laser intensity dependence of this process is measured.