The self‐bias voltage on the rf electrode is used to obtain information concerning negative ion current in rf magnetron sputtering from a single ceramic target. Measurement is made as a function of rf power for YBaCuO and other perovskite targets in an Ar+O2mixture plasma. The self‐bias voltage initially increases with increasing rf power, and then passes through a maximum and decreases. This abnormal relation between self‐bias voltage and rf power is enhanced with increasing O2/Ar ratio in the Ar‐O2mixture plasma and also for Ba‐containing targets. The bias voltage drop is consistent with the composition deviation of on‐axis deposited films and is attributed to negative ion formation at the target. Therefore negative ion formation can be indirectly monitored by the target self‐bias voltage.