Since the discovery of the crown ethers supramolecular chemistry has been dealing with molecular containers. Over the decades, the flat crown ethers have given way to three-dimensional shapes including cryptands, cavitands and ultimately capsules, designed to encapsulate molecular hosts. Now the emphasis is shifting from structure to function, as researchers are trying to make the nanoscale capsules work for them. This is possible through the absolutely unique class of polyoxometalates. In solutions of oxoanions of the early transition metals - in particular molybdates and tungstates - an enormous variety of compounds can be formed by linking together metal-oxide building blocks. In this dissertation we investigated the host-potential of some polyoxometalates and their formation stability.