Experimental approaches for recording ac impedance data are discussed in this paper. Considerations of the high frequency performance of potentiostats and reference electrodes are given and experimental results are presented. Additional methods for analysis of experimental impedance data are discussed, with emphasis on the CIRFIT computer program in which a semicircle is fitted to the experimental data in the complex plane. The systems Cu/3.5% NaCl + BTA and Al 2024/3.5% NaCl are discussed in more detail. Pitting of the Al alloy in neutral NaCl leads to a second time constant at low frequencies. Results for iron and Al 2024 in tapwater show that the low conductivity of this test medium does not present particular problems with the recording of ac impedance data.