Abstract The sensitized photoisomerization of stilbene was studied with nanosecond time-resolved absorption spectroscopy at room temperature. In non-viscous solvents (heptane, benzene, and ethanol), the transient spectrum does not depend upon a starting material being either cis- or trans-stilbene. The kinetic analysis of the transient spectrum due to stilbene shows that the observed spectrum consists of the Tn-T1 absorption spectrum of trans-stilbene and that of twisted triplet (phantom triplet) stilbene. The lifetime of the twisted triplet stilbene is determined to be approximately 120 ns. In a viscous solvent (t-butyl alcohol), the transient behavior of triplet stilbene is dependent upon the starting materials being either cis- or trans-stilbene. The contribution of the twisted triplet stilbene to the transient spectrum is larger for the case starting from cis-stilbene than for the case starting from trans-stilbene.