The state of the amphiphilic triblock copolymer poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) L64, (EO)(13)(PO)(30)(EO)(13) (EO = ethylene oxide, PO = propylene oxide) in aqueous solution at concentrations below the critical micelle concentration (cmc) or temperatures below the critical micelle temperature (cmt) has been investigated by fluorescence probing using the fluorescent probe pyrene (fluorescence intensity ratio I-1/I-3, and fluorescence lifetime tau), ultrasonic absorption and conductivity. The effect of addition of sodium dodecyl sulfate (SDS) on the these properties was also investigated. The results suggest that at concentration below the cmc and at temperatures below the cmt, L64 aggregates under the form of small short-lived oligomers that are capable of binding pyrene and SDS. These oligomers probably involve the more hydrophobic components contained in the L64 sample investigated, which is polydisperse in composition like all amphiphilic block copolymers.