Supercritical and subcritical CO2-based mixtures are being considered as environmentally benign media for photoresist and plasma etch residue removal in electronic device manufacture. Despite many attractive features, supercritical CO2 has little solvating power for photoresist or inorganic materials. Addition of a basic modifier, such as tetramethylammonium hydroxide (TMAH), to CO2 can yield a mixture suitable for residue removal. This work investigates chemical reactions between TMAH and CO2 to gain insight into the cleaning efficiency of subcritical CO2 mixtures containing TMAH. Results suggest that although tetramethylammonium carbonate/bicarbonate mixtures form, the cleaning ability of TMAH based mixtures in the liquid phase is not significantly affected. (C) 2003 The Electrochemical Society.