High-sensitivity differential scanning calorimetry (HSDSC) has been used to investigate, in detail, phase transitions in dilute aqueous solutions of poly(oxypropylene), POP. The calorimetric traces are distinctly asymmetric, which suggests an aggregation process though the reported contemporaneous appearance of clouding indicates that phase separation is occurring. Two equilibrium schemes are proposed to account for the HSDSC output : monomer reversible arrow aggregate reversible arrow separate phase or monomer reversible arrow; separate phase. The first proposes that aggregation is a precursor to phase separation. The second ignores this possibility. Model simulations of the HSDSC output of both phase separation and aggregation have been performed and compared with systems which are believed to either aggregate or phase separate. The data appear to be more suitably described by the scheme which has aggregation as a midway step. Model fitting to the aggregation model of the apparent excess heat capacity-temperature function has allowed evaluation of the van’t Hoff enthalpy. The effects of molecular mass, concentration, and scan rate on the characteristic thermodynamic parameters of the POP transitions have been evaluated. In addition the results have been compared with thermodynamic parameters obtained from a study of aqueous solutions of two poly(oxyethylene)-poly(oxypropylene) -poly(oxyethylene) (pluronic) block copolymers in order to investigate the effect of poly(oxyethylene) on the phase transitions. HSDSC model fitting suggests that the aggregation process involves several molecular clusters. The number of clusters and indeed the number of molecular chains comprising a cluster are dependent upon the molecular weight of the poly(oxypropylene) and, for the pluronics, on the presence of poly(oxyethylene) blocks. These numbers however appear to be relatively insensitive to changes in concentration. Finally an enthalpy-entropy compensation plot indicates that the underlying solute-solute and solute-solvent interactions which give rise to the phase transitions are the same regardless of sample.