The influence of the lipidic phase structures on their susceptibility to solubilization by the nonionic detergent C12E8 has been explored. To this aim, a phase diagram for dimyristoylphosphatidylcholine/ cholesterol in excess water has been constructed in which phase boundaries were derived from high-sensitivity differential scanning calorimetry. Six different lamellar phases can be obtained with this system, namely crystalline (L-c＇), gel (L-beta＇ or L-beta), rippled (P-beta＇), liquid crystalline or fluid (L-alpha), gellike liquid ordered (L-o beta), and fluidlike liquid ordered (L-o alpha). The solubilization of samples in each of these phases by C12E8 has been studied through changes in suspension turbidity under equilibrium conditions and also using a stopped-flow time-resolved technique. We find that variations in temperature and cholesterol content within a single phase can affect the equilibrium and kinetic parameters of detergent solubilization somewhat, particularly in the former case. However, much larger variations in the equilibrium and kinetics parameters of C12E8 solubilization are noted between different phases, particularly those phases containing cholesterol. In general, the presence of cholesterol potentiates the solubilization of DMPC vesicles at lower temperatures and inhibits their solubilization at higher temperatures. Moreover, in the more fluid phases (L-alpha for DMPC alone or L-o beta and L-o alpha for DMPC-cholesterol mixtures), vesicle turbidity was not affected by detergent concentration until concentration near the solubilization concentration was reached, at which point an increase in turbidity attributed to vesicle lysis and reassembly occurs prior to vesicle solubilization. In contrast, this effect is markedly reduced in the various gel phases of DMPC alone, where vesicle turbidity generally decreases monotonically with detergent concentration. Cholesterol-induced liquid crystalline-like liquid ordered phases (L-o alpha), that are presumed to coexist with the L-alpha phase in animal cell membranes, are much more resistant to solubilization than the predominant fluid disordered L-alpha phase, present in such membranes.