Ultrathin multilayer films of a novel molybdenum(VT) polyoxometalate cluster ((NH4)(21)[H3Mo57V6- (NO)(6)O-183(H2O)(18)]) (Mo-57) and poly(allylamine hydrochloride) (PAH) have been prepared by the consecutive stepwise adsorption of Mo-57 and PAH from dilute aqueous solution. The Mo-57/PAH multilayer films have been characterized by optical spectroscopy and small-angle X-ray reflectivity (XR) methods. UV-vis measurements reveal regular film growth with each Mo-57 adsorption. The average Mo-57 surface density was found to be (1.4 +/- 0.4) x 10(13) clusters per cm(2), corresponding to an average surface coverage of 56 +/- 12%. XR experiments confirm uniform film growth, with the film thickness increasing with each Mo-57 adsorption step. The average thickness for the Mo-57/PAH layer pair was determined to be 0.8 +/- 0.1 nm. The Mo-57 density in the film can be readily controlled by varying the polyelectrolyte interlayer separation between each Mo-57 layer, and the total film thickness can be controlled by altering the number of adsorption cycles.