The effect of N-dodecyl-N,N-dimethylamine N-oxide (C12NO) on the unilamellar liposomes prepared from egg yolk phosphatidylcholine (EYPC) was studied using turbidimetric and fluorescence spectroscopic methods. The concentrations of C12NO causing saturation and total solubilization of membrane were evaluated turbidimetrically. Liposomes filled with hydrophilic fluorescent probe calcein were used for the fluorescence leakage measurements. Dependence of fluorescence intensity on C12NO concentration can be divided into three linear sections. The turning points between individual sections represent two concentrations of C12NO which induce formation of small defects, specifically large pores in liposomal membrane. These pores are wide enough to enable calcein to be washed out from liposomes completely. We compared results obtained by the two methods used. Measurements of fluorescent probe leakage showed that free diffusion of calcein through pores in EYPC bilayer was observed at such C12NO concentrations when the form of liposome is still preserved and the saturation of bilayer is not yet finished. Higher C12NO concentrations are needed for solubilization, a phase transition from bilayers to micelles.