The objective of this research is the characterization of ringing in HCCI engines based on in-cylinder ion signal measurements. A correlation is identified to quantify ringing intensity from ion signals by comparing ion and pressure signal characteristics under ringing conditions in an HCCI engine. The maximum ion rise rate (dIon/dtmax) is shown to be an excellent indicator of the maximum pressure rise rate (dP/dtmax), a factor which is very important to measure ringing intensity. The effects of changing bias voltage and ion sensing resistors are also explored for their effects upon the ion ringing intensity. The results show that the ion ringing intensity correlation is accurate at quantifying ringing across a range of HCCI engine operating conditions, including various equivalence ratios, combustion timings and intake pressures.