Ultrathin rhodium films with a thickness ranging from 1 to a few monolayers were deposited on a single-crystal Ru(0001) surface in order to investigate the oxidation behavior of ultrathin epitaxial films on a dissimilar substrate. It is found that rhodium grows on Ru(0001) initially layer by layer, adapting the in-plane lattice parameters of Ru(0001). When exposing Rh films to oxygen environment (similar to 4.8x10(6) L O-2 exposure) at 660 K, 2-4 ML Rh films form a surface oxide composed of (9x9) O-Rh-O trilayers. Quite in contrast, oxidation of the 1 ML Rh/Ru(0001) film leads to a poorly ordered oxide with a rutile structure reminiscent of RuO2(110) on Ru(0001). The oxidized 1 ML Rh/Ru(0001) film contains much more oxygen than the oxidized thicker Rh films. Lower temperatures (535 K) and high doses of oxygen lead to a (1x1)-O overlayer on the 1 ML Rh/Ru(0001) surface, whose atomic geometry resembles closely that of the (1x1)-O phase on clean Ru(0001).