An in situ monitoring system for plasma-induced reactions in ethanol was developed by infrared absorption spectroscopy in multiple internal reflection geometry. We monitored in situ the preferential decrease in the IR peak intensity of CH3 and OH groups in liquid ethanol. In addition, we monitored in situ the preferential inclusion of OCH3 and free OH groups deposited on a multiple internal reflection prism. Unlike optical emission spectroscopy, the IR-based technique detects non-light-emitting compounds. It presents as a powerful alternative tool for monitoring liquid-phase plasma reactions.