Abietic acid is a major component of the oleoresin synthesized by many conifers and constitutes a major class of environmental toxic compounds with potential health hazard to animal, including human, and plant life. Being an amphipathic molecule, the study of the influence of abietic acid on the structure of membranes would be important to get insight into the mechanism of toxic action of the molecule. The interaction of abietic acid with model membranes of dipalmitoylphosphatidylcholine (DPPC) and dielaidoylphosphatidylethanolamine (DEPE) has been studied by differential scanning calorimetry and 31P-nuclear magnetic resonance spectroscopy. It has been found that abietic acid greatly affects the phase transition of DPPC, shifting the transition temperature to lower values, giving rise to the appearance of two peaks in the thermogram and to the presence of fluid immiscible phases. In a similar way, the phase transition of DEPE, in the presence of abietic acid, was shifted to lower temperatures, and two peaks appeared in the thermograms. The temperature of the lamellar to hexagonal H(II) phase transition was also decreased by the presence of abietic acid, but phase immiscibilities were not detected. The possible implications of these effects on the action of abietic acid on biological membranes are discussed.