Presently, a major factor restricting the achievement of higher power outputs from the CO2 laser is the occurrence of plasma instability. This instability may manifest itself in two forms causing either discharge striations (ionisation instability) or, more importantly, discharge arcing (thermal instability). Recent experimental and theoretical work, has identified these instabilities with the electron and ion kinetics of the CO2 laser discharge. The CO2 laser discharge, its development for high power output and the conditions leading to the two types of instability are reviewed. The requirement for full data on electron and ion processes for discharge modelling is clearly established. The techniques involved in mass spectrometric analysis of ions in gas discharge plasmas are reviewed and subsequently applied to the CO2 laser discharge. The major processes affecting the positive ion species in CO2 laser plasmas are determined in this way. The important negative ion species in the CO2 laser plasma are most easily and comprehensively determined by computational methods. These methods have allowed considerable insight into the dominant negative ion processes of the CO2 laser discharge. By application to specific laser situations correlation has been shown to exist between ion densities and the onset of experimentally observed discharge instability. The elucidation of the main ion processes, both by mass spectrometer and computer, has contributed detailed information for modelling of the CO2 laser discharge, and its instabilities, in a variety of configurations.