The frequency dependence of the impedance of the anodic tilm on Zr has been determined using UC impedance spectroscopy. This technique has been used to provide an in situ monitor of the thickness and resistance of the lilm during formation. The data suggest that the film can be modelled adequately by a parallel RC combination, with an additional series resistance corresponding to the solution resistance. A better fit was obtained using a constant phase element circuit, with a phase angle varying from 84" for thin films to 90" for thicker films. If 4 = 84", the thickness measured by a standard capacitance bridge would be expected to vary by a factor of 1.66 over a frequency range of 0.5-1000 Hz. The inverse of the capacitance (from fits to the parallel RC circuit) was observed to increase linearly with anodic.charge for all solutions and charging rates used in this work. The efficiency of Slm formation calculated from this data was < 100% for charging rates of 0.05 and 0.5 mA cn?, while it was close to 100% for a charging rate of 5 mA cn-', in borate and phosphate solutions. The efficiency exceeded 100% in sulphate solution at the high charging rate, presumably due to incorporation of sulphate ions from the electrolyte. The parallel resistance of the anodic film on Zr increased with open circuit exposure time, while the capacitance was unchanged, suggesting that the film is becoming more defect-free, but not thickening during the open circuit exposure. The air-formed film on Zr after the HF etch was about 5 nm in thickness.