The aggregation properties of ethylene oxide/propylene oxide triblock polymers in water have been investigated using the fluorescence probe molecule, sodium 2-(N-dodecylamino)naphthalene-6-sulfonat (C12NS). A blue shift in the C12NS emission maximum is observed as a function of block copolymer concentration and is associated with the formation of aggregates in solution. EO/PO block copolymer aggregation, as measured by the C12NS probe technique, was found to occur at lower concentration as the molecular weight of the hydrophobe (PO) was increased, as the hydrophilic-lipophilic balance (HLB) was reduced and as the temperature was increased. These results are discussed in light of the currently accepted model for nonionic surfactant aggregation in water : the critical micelle concentration paradigm.