UV-visible absorption and steady-state and time-resolved fluorescence techniques were used to investigate the aggregation behavior in Y-type hemicyanine Langmuir-Blodgett (LB) multilayers. Hemicyanine molecules formed H aggregates in LB films leading to blue-shifts of the absorption and fluorescence bands with respect to those of monomeric hemicyanine in chloroform solutions. The aggregates in LB films were dissociated by mixing with arachidic acid. The measurement of fluorescence decay profiles could be used to quantitatively characterize the lifetime, weights and size of aggregates.