It has been demonstrated that sufficient numbers of mass-selected cations can be directly deposited into growing inert gas matrices at low temperatures for spectroscopic analysis. However, the mechanism that must exist to maintain electrical neutrality in the matrix is not understood, We report here the direct observation of counterions in matrices formed during the codeposition of mass-selected cations and neon. The addition of carbon dioxide to the neon matrix gas during deposition of a mass-selected cation beam results in detectable quantities of CO2.-. The direct measurement of negative charges, as positive ions impinge on a cold target, provides insights into the mechanism through which charge neutrality is maintained in this experiment. Negative charge formation is strongly dependent on the temperature of the metallic surfaces surrounding the matrix window and the presence of condensable gases on these surfaces.