Refers to a gas or liquid that converts oxides from surfaces into carbon dioxide and water. This can be an acid or activated gases. Activation usually takes place through temperature or plasma energy.

During the reduction reaction, the surface oxide decomposes together with the reducing agent to form CO₂ and H₂O. What remains is the bare metal. The reducing agents used in electronics production are acids in fluxes such as carboxylic acids and, in flux-free soldering, formic acid or hydrogen.