The compounds and complexes of vanadium are used to treat diabetes and cancer. Research on the effectiveness and mechanism of action of new derivatives of vanadium, and their toxicity is currently very intense. The research shows that the vanadium(IV) acetylacetonate complex [VO(acac)(2)] shows a synergism with insulin in treating diabetes, high pharmacological activity and low toxicity. In order to improve the effectiveness of drugs and minimize their toxicity, the active compounds are often closed in the liposome membranes. The objective of the work was preparation of bilayer liposomes from egg yolk phosphatidylcholine (EYPC), closing the complex VO(acac)(2) in these membranes and estimating the concentration of vanadium complex after incorporation into liposomes membranes. Due to the paramagnetic properties of vanadium(IV) the concentration of this metal complex can be determined directly by EPR. Entering the spin label CTPO in the water phase into the studied arrangement allows for the indirect measurement of the concentration of complex, on the basis of changes of the EPR spectrum of the spin label caused by the presence of the vanadium(IV) complex. In the work the dependence of the a parameter based on the analysis of CTPO EPR spectra on the concentration of VO(acac)(2) was determined. To demonstrate the presence of the complex in the membrane directly by measuring the EPR sulfate(IV) sodium was used in order to remove the EPR signal of vanadium(IV) from the water phase. The presence of vanadium(IV) in the membrane was also demonstrated indirectly using a spin label 12-SASL. Based on the results of EPR spectroscopy the concentration of the complex in the membrane was determined together with the partition coefficient of VO(acac)(2) between the membrane and outer water environment of the membrane.