Abstract A differential ac calorimeter has been developed to investigate the phase transition behavior of liquids of biological interest, especially suspensions of liposomes and other model membranes. The sensitivity of the calorimeter is independent of the temperature scan rate and scan direction. Therefore, the heat capacity can be measured during heating scans, cooling scans, and at constant mean temperature. Thus, the time-dependent behavior of the heat capacity can be measured. A rather massive sample cell has been used to minimize problems with hermetically sealing the cell and maintaining a constant thermal path between the heater and thermometer. As the heat capacity of this cell is large compared to that of the sample, a differential method has been used to remove the heat capacity of the cell. Results are presented for heating, cooling, and quasi-isothermal studies of the main phase transition in dimyris-toylphosphatidylcholine liposomes. Heating scans at 3°C/h are in excellent agreement with conventional scanning calorimetry. Cooling scans exhibit a supercooling of up to 0.5°C at this transition, confirming the first-order nature of the transition. Quasi-isothermal measurements for up to 20 min in the transition region show that there is no observable, instrumental distortion of the transition at these low scanning rates.