Heats of dilution of aqueous solutions of dihexanoylphosphatidylcholine were determined by use of a flow microcalorimeter to monitor an exponential dilution gradient. Three different models of micelle formation were tested: monomer in equilibrium with micelles of fixed size, with micelles of varied size, or with small aggregates and micelles. The heat of dilution data for low solute concentration could be fit only by assuming the existence of premicellar aggregates. The critical micelle concentration determined calorimetrically is 0.016 +/- 0.002 M and is independent of the model. The enthalpy change for transfer of monomer into the micelle is 1.6 +/- 0.2 kcal/mol; about one-third of this heat effect is produced in formation of the premicellar aggregation. Comparison of the calorimetric measurements with results obtained by using other methods indicates the complexity of the micellization process.