A recent Letter published in this journal presented very interesting experimental results, which undoubtedly provided a key to elucidate the J-aggregation mechanism of a thiacarbocyanine dye. The major conclusions of that Letter are significant and acceptable able. However, the calculation in the Letter cannot be reproduced when an usual quasi-equilibrium approximation is adopted, although the Letter assumed a quasi-equilibrium state. The mathematical details of the adopted assumptions should have been described and justified clearly. (C) 2004 Elsevier B.V. All rights reserved.