The ionization sensor is an elec. probe for diagnostics in internal combustion engines. Laser-induced fluorescence (LIF) imaging of fuel, hydroxyl (OH), and nitric oxide (NO) distributions has been employed to extend our knowledge about the governing processes leading to its signal. By monitoring the flame propagation in quiescent and turbulent mixts., the cycle-to-cycle variations in the early sensor signal was attributed to the stochastic contact between flame front and electrodes. An anal. of the relationship between gas temp. and sensor current in the post-flame gas suggests a dominant role of alkali traces in the ionization process at the conditions under study. Significant cooling of the burned gas in the vicinity of the electrodes was obsd. in quiescent mixts. Imaging of the post-flame gas in turbulent combustion revealed moving structures with varying NO and OH concns., which were identified as sources of variation in the sensor current.