For the liquid-phase photolytic denitrogenation of the stereolabeled DBH derivative exo-d(2)-diazabicyclo[2.2.1]heptene (exo-d(2)-1), the k(inv)/k(ret) ratio of the inverted [2(inv)] and retained [2(ret)] housanes (bicyclo[2.1.0]pentanes) depends on the viscosity of the medium. For this purpose, the viscosity was varied by changing the solvent(various alcohols and diols, n-hexane, and acetonitrile) at constant temperature and by changing the temperature (-50 to +100 degreesC) in one single solvent, namely n-butanol. This viscosity effect is consistent with a stepwise denitrogenation mechanism in the liquid-phase photolysis of DBH, which proceeds through an unsymmetrical, nitrogen-containing transient, namely the singlet diazenyl diradical. The simple free-volume model adequately accounts for the observed viscosity behavior of the ki,,lk,, ratio in terms of frictional effects. The temperature dependence discloses a small but measurable difference in the internal activation energies for the inversion and retention processes of the proposed diazenyl diradical.