Abstract: A non-intrusive means and method are disclosed for monitoring the interelectrode gap in a reaction chamber containing a pair of variably spaced, nominally parallel reaction-sustaining electrodes. An example is given in which the chamber is a plasma enhanced chemical vapor deposition chamber. The electrodes are treated as a parallel plate capacitor in order to measure the gap therebetween as well as the parallelism of the electrodes without requiring that the chamber be opened. A calibration curve is prepared by ascertaining known values of gaps by use of spacer gauges and then measuring and storing the corresponding values of the gaps and the capacitances. Once the calibration curve is established and stored, the spacing between the electrodes can be checked from time to time, as needed, without opening the chamber. It is only necessary to measure the capacitance existing and then refer the measured value to the stored values to read out the corresponding gap value.