(E)-N-Methoxy-1-(2-anthryl)ethanimine ((E)-I), ArC(CH3)=N-OCH3 (Ar = 2-anthryl), undergoes one-way adiabatic s-trans --> s-cis internal rotation around the single bond connecting the 2-anthryl group and the unsaturated carbon in the tripler state as well as in the singlet excited state at ambient temperature. Below 173 K, the internal rotation takes place only in the triplet state. At 149-173 K, the (3)(s-trans)* --> (3)(s-cis)* internal rotation, accompanying the change of the T-T absorption from lambda(max) = 438 nm to lambda(max) = 442 nm, proceeds with an activation energy of 4.6 kcal mol(-1) and a frequency factor of 8.9 x 10(11) s(-1) in methylcyclohexane. The potential energy surface of s-trans --> s-cis conversion is depicted based on the observed quantities.