A system for both ignition and ion current measurement was designed and set up at Istituto Motori. Particular attention was paid to the problem of dissipating the residual energy stored in the ignition coil, reducing the electromagnetic interferences and especially improving the response of the measurement system. In order to assess the capability of the ion current signal to give reliable and accurate information for knock detection, a number of tests were carried out at full load on a commercial PFI four-cylinder engine, at various air/fuel ratios and spark timings. Some knock indices based on the ionization signal, both band pass filtered and non-filtered, were introduced, in particular: the Amplitude of the Second Ionization Peak (ASIP), the Mean not-filtered Ionization Current signal (MIC), the Maximum Amplitude of Ionization Current signal Oscillation (MAICO), the Integral of Modulus of filtered Ionization Current signal Oscillation (IMICO). The thresholds of the MAPO (Maximum Amplitude of Pressure Oscillation) knock index, based on the band pass-filtered pressure signal, were used to classify no-knocking and light-knocking cycles. A criterion based on the Percentage of Knocking Cycles was proposed to evaluate the MAPO-equivalent thresholds of the considered indices to classify the knock type. In particular, thresholds for the MAICO and IMICO indices were found, allowing a fairly good classification of no-knocking and light-knocking cycles based on ion current measurement.