Polyoxometalates (POMs) are a unique class of metal oxides, being of huge interest for the catalysis society. Co(II)-substituted phosphomolybdate POMs (namely H7[PCoMo11O40], H11[PCo2Mo10O40] and H15[PCo3Mo9O40]) have been successfully synthesized for the first time. The structure of the new Co(II) substituted POMs was solved by single-crystal X-ray diffraction as well as vibrational (FTIR and Raman) and magnetic (NMR and EPR) spectroscopy and found to be of the Keggin-type. The Co-POMs were then applied as molecular catalysts for the hydroformylation of 1-hexene in a thermomorphic solvent mixture of water and 1-butanol. This is the very first example of a 1st row transition metal POM acting as a hydroformylation catalyst achieving 46% heptanal yield. The best results were obtained using H11[PCo2Mo10O40] as a catalyst at 140 °C, 150 bar CO/H2 1:2, and a reaction time of 4 h, paving the way for new POM-based catalysts in hydroformylation reactions.