Field crystallisation during the anodisation of niobium can be reduced to a minimum by employing a phosphoric acid electrolyte at low temperature. For certain other electrolyte systems, reduced crystalline growth rates are obtained by increasing the solute concentration or by adding either phosphate or ethylene glycol.Porous anodes were prepared from niobium powders of nominal particle size 8μm or 10μm using sintering temperatures of 1400–1800℃. The capacitance of these bodies was in the range 4,600–10,000μF Vgm-1. after anodising at 20℃. Shelf tests on experimental capacitors filled with a concentrated sulphuric acid electrolyte indicated a maximum working temperature of 85℃. Although chemical compatibility between the dielectric oxide and the sulphuric acid was maintained up to 125℃, thermal effects on the oxide at this temperature resulted in unacceptably large increases in capacitance and leakage current. Endurance tests at 85℃ of capacitors rated at 35V/200μF or 63V/60μF showed good stability of capacitance and dissipation factor over a period of 3000 hours. Final leakage currents were in the region of 10-4μAμF-1V-1 at 85℃ and crystalline growth was greatly restricted.