The systems Nb2O5-Ta2O5 and Nb2O5-V2O5 were investigated using thermal analysis, X-ray powder diffraction and thermodynamic simulations. Solid solution formation is possible for both systems; furthermore, both contain one intermediate compound, VNb9O25 or Ta2Nb4O15, respectively. Phase relationships for pure niobium(V)oxide and tantalum(V)-oxide were studied under ambient pressure. It was found that both compounds can occur in two stable solid modifications. For niobium(V)-oxide this are the monoclinic high-temperature modification (H-Nb2O5) and an orthorhombic low-temperature modification (T-Nb2O5) and for tantalum(V)-oxide a tetragonal high-temperature form (alpha-Ta2O5) and an orthorhombic low-temperature form (beta-Ta2O5). Based on these results, crystal growth experiments with various compositions from both systems were carried out using the optical floating zone (OFZ) technique.