The viscosity dependence for the excitation step of the chemically initiated electron-exchange luminescence (CIEEL) triggered from a novel bicyclic furan-annelated dioxetane has been studied. The observed small but mechanistically significant enhancement of the CIEEL efficiency with increasing viscosity is rationalized in terms of the free-volume model, in which the molecular bond-rotation process that leads to the ground-state product is frictionally impeded and the chemiexcitation yield is higher.