The effect of Keggin heteropolyoxotungstates (XW12O40n- with X = H-2, P, Si, B or CO) on Langmuir films has been studied for monolayers of DODA (dimethyldioctadecylammonium) and DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine). Marked modifications of the compression isotherms have been observed when the Keggin anions mere dissolved in the subphase : this demonstrates that the polyanions interact with the monolayer. Langmuir-Blodgett (LB) films have been readily obtained from these systems (even with DPPC) for a particular range in polyanion concentration. X-ray diffraction and infrared dichroism experiments have shown a well-defined lamellar structure for these built-up films as well as the presence within the LB films of polyoxometalates organized in monolayers. Control of the Keggin polyanion amount in the multilayers is made possible by mixing DODA with a negatively charged lipid, which modifies the global electrical charge of the Langmuir film. Such an organic-inorganic system leads to new ultrathin materials having various properties related to the selected polyanions.