By measuring the transition peak using differential scanning calorimetry in the heating and cooling modes, it has been confirmed that the nematic-to-isotropic liquid phase transition of 4,4'-azoxyanisole is of first order, but with some underlying additional phenomenon. As a consequence, this phase transition can only be used under certain precautions for temperature calibration of scanning calorimeters in the cooling mode. (c) 2006 Elsevier B.V. All rights reserved.