The aggregation behaviour of two ethylene oxide-propylene oxide-ethylene oxide block copolymers [PE182:(EO)(6)-(PO)(34)(EO)(6) and PE333:(EO)(17)(PO)(60)(EO)(17)] has been investigated using high sensitivity differential scanning calorimetry. The thermal data obtained was examined using previously outlined procedures [I. Patterson, J. Armstrong, B. Chowdhry, S. Leharne, Langmuir 13 (1997) 2219; B.Z. Chowdhry, M.J. Snowden, S.A. Leharne, J. Phys. Chem. 101(1997) 10226]. These analysis routines have revealed the existence of a small dissociation transition within the thermal envelope of the overall aggregation transition. The transition appears to be associated with the thermally induced break-up of large aggregates and occurs simultaneously with or slightly before the micellar aggregation of the block copolymers. A reasonable linear relationship exists between the van't Hoff enthalpy and T-1/2 (the temperature at which the transition is half completed) using data obtained for both copolymers, which suggests that the origin of the dissociation transitions in both copolymers is the same.