Anodization current efficiency measurements were used to determine the oxygen content of sputtered tantalum films. Films consisting mainly of b.c.c. Ta had less than 5 at.% oxygen while 'normal' β-Ta contained 10–15 at.% oxygen. Films with a more negative temperature coefficient of resistance had a higher oxygen content.The field strength required to cause oxide growth on 'normal' β-Ta was lower than for b.c.c. Ta films or for oxygen rich β-Ta films. The cause of these changes in field strength is not known. The dielectric constants of the oxides grown on all films were the same, but depended on the measurement conditions and history of the oxide.