When only one of the two rotamers of 3-pyridinecarboxaldehyde (3-PCA) vapor was excited into the S-1(n, pi*) state, the T-1(n, pi*) phosphorescence bands of both of the two rotamers were observed with almost the same band intensity ratio as that observed under thermal equilibrium condition. When the S-0 --> S-1 excitation spectrum was measured by monitoring the phosphorescence band of only one of the two rotamers, the excitation bands of both of the two rotamers were observed with almost the same band intensity ratio as that observed under equilibrium condition. These observations show occurrence of the intramolecular CHO group rotation in the excited state. (C) 2008 Elsevier B. V. All rights reserved.