Alkali metal salt modified SAP (supported aqueous-phase) rhodium catalysts prepared by coimpregnation method using alkali metal chloride were found to be active and selective for olefin hydroformylation. The salt addition promoted the formation of aldehydes with high selectivity, the aldehyde yield being increased more than 2.5 times at a proper salt/Rh ratio. Changes in stretching frequency of the carbonyl species were detected during ethene hydroformylation, which appeared at ca. 1625 cm-1 on the non-modified SAP catalyst sample, while at ca. 1586 cm-1 on the KCl-modified one, as shown by in situ IR spectroscopy. The results of a deuterium isotope effect experiment showed that the hydroformylation rate for aldehyde formation on SAP rhodium catalyst under atmospheric pressure of CO/D2 was about 1.3 times faster than that under CO/H2, implying that the rate-determining step involved in aldehyde formation is most probably a step related with hydrogen. The role of the alkali metal salt is discussed in relation with the reaction mechanism.