The positive effects of Selenium added to Rh/SiO2 catalysts in ethene hydroformylation were studied, and compared to the case of Se-Rh/ZrO2 catalysts. The addition of Se promoted propanal formation in the case of Rh/SiO2 in contrast to preferable propanol formation on Se-Rh/ZrO2. The active site structure was monitored by Se K-edge EXAFS (Se-Rh distance 0.241 nm). The XPS peaks of Se 3d(5/2) exhibited the valence state of -1 ~ -0.5 of Se and nearly metallic state of Rh. The intermediate structure of propanal formation is discussed in terms of the effects of selenium.