Solid-solution Nb-O films, containing up to 50 at% oxygen, prepared by magnetron sputtering were used to investigate the influence of the oxygen on field crystallization during anodizing at 100 V in 0.1 mol dm-3 ammonium pentaborate electrolyte at 333 K. The findings reveal that field crystallization is hindered dramatically by addition of 20 at% oxygen to the substrate, while no crystallization occurs for an Nb-50 at% O substrate. Prior thermal treatment accelerates field crystallization of niobium, but not of the Nb-50 at% O substrate. The thermal treatment is considered to promote generation of pre-cursor sites for crystal nucleation. However, sufficient oxygen in the substrate may restrict pre-cursor development and/or reduce the compressive stresses in the amorphous anodic niobia that can facilitate crystal growth. copyright The Electrochemical Society.