The two synthetic sterol phospholipid hybrids DCholPC and PCholPC were investigated in monolayers at the air/water interface. Study was based on pi-A isotherm analysis complemented with application of grazing incidence X-ray diffraction. It was found that both compounds are capable of forming stable, highly condensed monolayers of a surface characteristics typical for sterols. GIRD studies show that the crystallographic area for DCholPC monolayer is very similar to that found for cholesterol (37.1 vs 38.0 angstrom(2)), while for PCholPC (28.8 angstrom(2)) it is significantly smaller as compared to area for the mixed Chol/DPPC 2/1 monolayer (33.4 angstrom(2)). In our study the problem of interactions between investigated sterol-phospholipid hybrids and natural membrane lipid components was for the first time analyzed in planar lipid systems. Studies on mixed monolayers showed that both hybrids, similarly to cholesterol, reveal a condensing effect toward DPPC acyl chains; however, DCholPC having two steroid moieties in the molecule was found to be more efficient. On the other hand, the sterol moiety and the hydrocarbon chain of PCholPC molecule are packed in the 2D crystalline phase extremely tight. Our studies showed that the investigated compounds can be applied as biocompatible components of stable liposomes.