It has been experimentally demonstrated that viscosity differentiates the extent to which inverted and retained housanes are formed in the liquid-phase denitrogenation of a substituted diazabicyclo[2,2,1]hept-2-enes (DBH)-type azoalkane, namely syn-2,3-diaza-4,7-dimethyl-1-ethyl bicyclo[2,2,1]heptene. The fact that the viscosity profiles coincide for the inversion/retention product ratio, assessed for a set of diverse solvents at constant temperature and through the temperature variation in a single solvent (n-butanol), shows that the temperature dependence of the stereoselectivity is controlled by bulk viscosity effects. The simple free-volume model rationalizes the viscosity behavior of the inversion/ retention ratio. Since the viscous medium differentiates between the inversion and retention reaction channels, we conclude that an unsymmetrical, nitrogen-containing intermediate, presumably the singlet diazenyl diradical, intervenes in the liquid-phase deazetation process.