The vesicle-micelle transition of Lipid bilayers induced by the addition of the nonionic detergent octyl-beta-d-glucopyranoside (OG) was studied with high-sensitivity titration calorimetry and spectroscopic methods. Sonified phospholipid vesicles composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were titrated into OG solutions with concentrations between 15 and 22 nM. The critical OG concentration for bilayer micellization was determined as c(D)* = 15.4 +/- 0.3 mM whereas the critical micellar concentration of pure OG was c(cmc) = 23.7 mM (in buffer). All calorimetric titrations were performed below the cme of pure OG solutions, The titration curves can be explained by a superposition of three processes, namely (i) a partitioning of surfactant molecules into the lipid membrane, (ii) a membrane micellization, and (iii) a membrane re-formation. The basic process is a partitioning of OG molecules into the phospholipid vesicles which occurs during the whole titration experiment. it can be described according to X-b = Kc(D,f) where X-b is the degree of binding (effective surfactant-to-lipid ratio) and c(D,f) is the equilibrium detergent concentration free in solution. The thermodynamic parameters were determined as K = 88 +/- 3 M-1 and Delta H-D(o) = 1.7 +/- 0.4 kcal/mol for the transfer of OG from the aqueous phase to the membrane (15 mM less than or equal to c(D)(o) less than or equal to 22 mM). OG partitioning entails a bilayer micellization in the initial phase of the lipid-into-detergent titration experiment. Bilayer micellization is an endothermic process with Delta H-mic = +1.85 +/- 0.1 kcal/mol lipid. With continuous addition of lipid, bilayer micellization comes to a halt and is replaced by the reverse process. The demicellization process is exothermic with Delta H-demic = -1.84 +/- 0.1 kcal/mol lipid, Micellization and demicellization are thus symmetrical processes, Once the free OG concentration falls below the critical limit of c(D)* = 15.4 nM, the titration curves follow the predictions of the partition model, A quantitative model is proposed to describe the excess heat, , of the bilayer reversible arrow micelle equilibrium. To a first approximation, follows the symmetrical mixing behavior of two closely related nonpolar liquids. Phosphorus-31 nuclear magnetic resonance and right-angle light scattering were used to monitor the structural changes of the OG/POPC mixtures. They confirm a phase boundary between the bilayer phase and the bilayer/micelle coexistence phase at c(D)* = 15.4 +/- 0.3 mM and X-b = 1.36 +/- 0.04. The phase boundary between the bilayer/micelle coexistence phase and the pure micellar phase is found For X-b greater than or equal to 2.8. The calorimetric titrations can be used to construct a rather precise phase diagram of the POPC/OG system as the discontinuities of the titration curves define the critical concentrations of detergent and Lipid at the phase boundaries.