Abstract The theory of the angular distribution of the photofragment resulting from weak predissociation in a diatomic molecule is worked out in the density matrix formalism. Special attention is given to the relationship between photofragment anisotropy, molecular polarization and fluorescence light polarization. The effect of a steady applied magnetic field is discussed and compared with classical Hanle effect. Application to the case of O 2 +, b 4σ R − state studied by fast ion beam laser spectroscopy (FIBLAS) is presented. Zeeman effect of the low J levels is observed in good agreement with theory and the angular distribution of the photofragments arising from a few selected Zeeman sublevels offers qualitative experimental confirmation of the theoretically predicted behavior.