Formylation of aryl iodides proceeded effectively in the presence of palladium nanoparticles on cobalt oxide under a syngas atmosphere to afford aldehydes up to 91% yield. A cooperative effect between palladium nanoparticles and cobalt species derived from the support was integral to efficient transformation. Both palladium and cobalt were revealed to exist as zero valent metals after H 2 treatment from X-ray absorption near edge structure and X-ray diffraction spectra. The catalyst could be reused at least 7 times without significant loss of activity.