Studies of inversion potentials have been carried out for gas‐phase 1,4‐dihydronaphthalene (DHN) using ultraviolet laserspectroscopy and ab initio quantum chemical calculations. The analyses of the experimental fluorescence excitation and dispersed fluorescence spectra in a supersonic free jet indicate that the equilibrium conformation of DHN is nonplanar in both the ground and lowest excited singlet states. However, the barrier to inversion is smaller than the energy of the zero‐point vibrational level, so that DHN behaves as a quasiplanar molecule. The ground‐state structure of DHN computed at the correlated MP2/6‐31G(d) level of theory is also nonplanar, with a dihedral angle of 148° and an inversion barrier of 0.5 kcal/mol. Comparisons are made for the series 1,4‐cyclohexadiene (1,4‐dihydrobenzene), DHN, and 9,10‐dihydroanthracene.