The variations of the equivalent parallel resistance and capacitance of anodic films formed on Ta and Nb in varying concentrations of H 2SO 4 suggest that the volume of film formed per coulomb is much greater in 100% acid, although the dielectric constant is substantially unchanged. The field required for growth on Ta in 100% acid is about 85% of that in aqueous solutions, where it is constant up to about 60% H 2SO 4. Slow conversion of films formed in 100% acid occurs when they are immersed in dilute H 2SO 4; this change is very rapid when charge is passed. Virtually no change occurs in the converse case, indicating that the change in film properties found during growth in 100% acid is a characteristic of the growth process. Marked corrosion or dissolution of the film material in 100% H 2SO 4 is apparent with Ta at temperatures much above 50°C, and above 25°C for Nb. The similarity of the behaviour of the loss factor, tan δ, with both metals, as the film thickness is increased, suggests that areas of high loss, but not of low dielectric constant, arise by the trapping of solution, and that these exert a predominating influence.