The luminescence of cyanine J aggregates was measured at room temperature. It is found that the J band luminescence of a cyanine dye J aggregate occurs effectively even though the excitation is on the low-energy side (0.12 eV) of the J band. It is established this is not due to the two-photon absorption. The lifetime of the J band luminescence with the low-energy excitation is 57 ps in contrast to 10 ps for band-to-band excitation. The rise time is also different: 6 ps for the low-energy excitation and less than I ps for band-to-band excitation. (C) 2004 Elsevier B.V. All rights reserved.