The poloxamer 284 (Pluronic P94) is a triblock copolymer of poly(ethylene oxide) and poly(propylene oxide). P94 and fractions of P94 obtained after centrifugation at temperatures where solutions contain both dissolved unimers and micelles have been investigated by differential scanning calorimetry, mass spectrometry, and NMR. The results show that the P94 sample is heterogeneous with respect to both chemical composition and molar mass. The first micelles formed, when the temperature is increased, contain poloxamers with a significantly higher propylene oxide content and a higher molar mass in agreement with theoretical predictions. The characteristic temperatures of micellization, sphere-to-rod, and phase separation transitions observed in thermograms are influenced by polydispersity, which results in broader transitions.