The rate of formation of diolein bimolecular lipid membranes (BLM) is measured by taking pictures of the expanding black area at different times. Total formation times, from the appearance of the first black spot until the completion of the BLM, are plotted versus temperatures; the curve shows that a phase transition occurs at about 13 °C. The variations of the black area radius versus time show first a transitory behaviour followed by a steady state. The constant rates corresponding to these steady states change steeply with temperature around 13°C. A model involving only interfacial tensions and viscosities is proposed to describe the transitory period. It implies, as a first approximation, the BLM border line to be a circle.