Lateral diffusion coefficient and order parameter measurements were made with pyrene excimer optical probes and fatty acid spin label probes respectively in pure dipalmitoyl phosphatidylcholine membranes and in membranes doped with tocopheryl acetate. The investigation shows, that the lateral diffusion coefficient for pyrene in dipalmitoyl pholphatidylcholine membranes is decreased whereas the order parameter of the fatty acid chains is slightly increased in the inner part of the membranes by the addition of tocopheryl acetate. The fluid-solid equilibrium phase diagram of dipalmitoyl phosphatidylcholine/tocopheryl acetate mixed membranes has been constructed from the measurements of the partition of (2,2,6,6-tetramethylpiperdine-1-oxyl) TEMPO spin labels between lipid and aqueous regions as function of temperature. In the membranes tocopheryl acetate induces a strong broadening of the temperature range of the phase transition. At low tocopheryl acetate concentrations dipalmitoyl phosphatidylcholine and tocopheryl acetate seem to be completely miscible in the solid and in the liquid crystalline state.