Upon photochemical nitrogen extrusion, azoalkane 1b yields the diastereomeric housane products syn-2b (inversion) and anti-2b (retention), whose syn-to-anti isomerization (k(iso)) is observable already at room temperature. From the similar viscosity dependence of the k(inv)/k(ret) data for the photolysis of the azoalkane 1 b and the k(iso) data for the thermolysis of the housane syn-2b, we conclude that these skeletal inversions are subject to frictional impediments and these determine the stereoselectivity.