A system and method for monitoring the conditions in a gas plasma processing system while varying or modulating the RF power (2A, 2B, 2C) supplied to the system, so that resulting signals of the electrical circuits of the system provide information regarding operational parameters of the system or the state of a process. Significant improvements in sensitivity and accuracy over conventional techniques are thereby achieved. In addition, the plasma processing system can be thoroughly tested and characterized before delivery, to allow more accurate monitoring by monitor controller (1) of and greater control over a process, thereby improving quality control/assurance of substrates (40) being produced by the system. The information obtained by the modulation technique can be displayed on a monitor screen, in order to allow an operator to accurately monitor the system/process and diagnose any problems with the system/process.