High-sensitivity differential scanning calorimetric (HSDSC) data obtained for mixtures of poly(oxypropylene) (POP) oligomers, of different molecular masses, in aqueous solution have been deconvoluted using a previously reported mass action thermodynamic model of aggregation (Armstrong, J.; et nl. J. Phys. Chem. 1995, 99, 4590) to investigate polymer aggregation/phase separation in these mixed systems, The data shows that POP oligomer solutions will phase separate at well defined temperatures as the POP solution is warmed. The data obtained shows no scan rate dependence which prompts the hypothesis that phase separation is adequately described by a nucleation and growth mechanism, It is concluded that the nucleation step is observable by HSDSC while the growth phase is calorimetrically silent. The model derived parameters indicate that the : phase transitions occurring at the lower temperature are only slightly modified compared to those values obtained for the respective single-polymer solutions, whereas the higher temperature transitions are markedly altered, A descriptive model is proposed which is used to explain the obtained results. Essentially the model suggests that the higher molecular mass component, which phase separate at lower temperatures, aggregates and phase separates independently of the lower molecular mass component. However, depending upon the molar ratio of the two components, the lower molecular mass component will, at the appropriate temperature; either join the already existing high molecular mass component aggregates or will form separate aggregate nuclei.