Dielectric properties of anodic oxide films formed on niobium in oxalic acid or succinic acid solution with addition of ammonium hydroxide were investigated with particular attention to the incorporated electrolyte species. The films’ capacitance increased considerably when the electrolyte pH was adjusted to more than 8 and the nitrogen content in the films increased concomitantly with increasing electrolyte pH. The anodic film formed in the oxalic acid solution showed the highest capacitance and the relative permittivity of the film was assumed to be 256. Impedance measurements indicated a monolayer in films formed in acidic electrolytes, whereas films formed in alkaline electrolytes were composed of two layers having different resistance. The outer layer thickness, accounting for approximately 65% of the film, was related to that of the layer containing nitrogen. Therefore, nitrogen incorporation was inferred as a cause of double layers in films formed in alkaline electrolytes, as reported in the case of alkaline phosphoric acid. The leakage current of the films was independent of the electrolyte species when measurements were conducted in the dark. However, under the room lighting, the leakage current of films formed in an electrolyte of pH 10 was approximately 10 times higher than that of films formed in acidic electrolytes. The photoreactivity is attributed to the semiconductor property of a niobium oxide film induced by nitrogen incorporation.