Real-time energy exchange among the three vibrational modes forming a triplet with frequencies omega(1), omega(2), and omega(3) satisfying a 'circa sum-frequency relation', namely omega(1) + omega(2) congruent to omega(3) in a cyanine dye molecule has been studied with a 7-fs pulse laser. There are two triplets with average frequencies of the components of (146, 279, 432 cm (1)) and (279, 432, 738 cm (1)). Even though the circa sum-frequency relation is satisfied, circa-conservation of the vibrational energy holds among the three components only in the latter triplet. The difference between the two is ascribed to the symmetry of the molecular vibrational modes. (C) 2008 Elsevier B. V. All rights reserved.