The anodic oxidation of niobium[1–4] is investigated in an electrolyte composed of pure, molten ortho-phosphoric acid, i.e. o-H3PO4.[5–7] In a range of applied potential of 2.5-60 V and for temperatures of 60-110°C, porous niobium oxide layers can be formed.Pore ordering and layer morphology depend on the anodization voltage, time and temperature.At 100°C and 5 V, vertically-oriented, defined amorphous nanochannels grow with an average diameter of 6-8 nm. Higher voltages (10-20 V) lead to a lower pore ordering and to a less defined morphology that resembles a “fish bone” structure. Interestingly, at 40-60 V and after a sufficiently long anodization time (≥ 30 min), we observe the formation of a partially crystalline Nb2O5 hierarchical structure. Such structure shows a bimodal pore size distribution, with some 100 nm wide main pores that branch out into ~ 10 nm sized nanochannels.We propose that high voltages (> 40 V) cause significant heating at the oxide/metal interface and a consequent partial crystallization of the anodic oxide. The main pore walls, partially crystalline and hence more stable in the anodizing environment, result from a gradual dissolution of the side nanochannels.[1] Q. Lu, T. Hashimoto, P. Skeldon, G. E. Thompson, H. Habazaki, K. Shimizu, Electrochem. Solid-State Lett. 2005, 8, B17–B20.[2] I. Sieber, H. Hildebrand, A. Friedrich, P. Schmuki, Electrochem. commun. 2005, 7, 97–100.[3] J. Choi, J. H. Lim, S. C. Lee, J. H. Chang, K. J. Kim, M. A. Cho, Electrochim. Acta 2006, 51, 5502–5507.[4] W. Wei, K. Lee, S. Shaw, P. Schmuki, Chem. Commun. 2012, 48, 4244.[5] M. Altomare, O. Pfoch, A. Tighineanu, R. Kirchgeorg, K. Lee, E. Selli, P. Schmuki, J. Am. Chem. Soc. 2015, 137, 5646–5649.[6] K. K. Upadhyay, M. Altomare, S. Eugénio, P. Schmuki, T. M. Silva, M. F. Montemor, Electrochim. Acta 2017, 232, 192–201.[7] K. K. Upadhyay, G. Cha, H. Hildebrand, P. Schmuki, T. M. Silva, M. F. Montemor, M. Altomare, Electrochim. Acta 2018, 281, 725–737.