An ac four-probe set-up was constructed to measure impedance spectra between 1 Hz and 100 kHz on solid electrolytes up to impedance values of 10 G Omega. The influence of the system on the measurement was analysed for the previous and the new set-up. A solution to minimise the source of errors by pre-amplifiers for current and voltage signals is presented. A new sample holder was constructed for best results in conjunction with the new Pre-amplifiers, The holder is designed for measurements on solid electrolytes over a wide temperature range. The performance of the improved set-up and the residual error sources are discussed. Measurements of impedance spectra on YSZ and BICUVOX.10 in a four-probe arrangement are given as an example.