The time course of the size transformation of sonicated small unilamellar dipalmitoylphosphatidylcholine vesicles at 23 degrees C has been followed with (31)P and (1)H nuclear magnetic resonance (NMR) spectroscopy. Comparison of these results with turbidity measurements indicates that vesicle aggregation, monitored by turbidity, and size transformation, followed by NMR, occur on distinctly different time scales. For lipid concentrations in the 5-50 mM range, aggregation takes place on a time scale of minutes, whereas size transformation takes many hours. Aggregation, unlike size transformation, can be reversed by increasing the temperature above the phospholipid phase transition temperature. Analysis of the (31)P NMR line shapes provides evidence for a model in which the small vesicles transform into the product vesicles [characterized by Schullery, S.E., Schmidt, C.F., Felgner, P., Tillack, T.W., & Thompson, T.E. (1980) Biochemistry 19, 3919] without producing significant amounts of intermediate-size vesicles. Kinetic analysis indicates that the size transformation is apparently second order. (1)H NMR data indicate that the rate of transformation is decreased if trivalent ions are added to the dispersions and also if the temperature is periodically increased above the transition temperature. Analysis of the latter experiment provides some evidence that vesicle aggregation is a necessary precursor to size transformation. It was also found that increasing the average vesicle size decreases the extent of transformation.