An improved method for the chlorine dioxide treatment of oil and/or gas well bores is provided, which eliminates the need for costly and complex chlorine dioxide-generating equipment and yields very high concentrations of chlorine dioxide down hole. The preferred method involves the introduction of reactant(s) into a well and causing a reaction to generate chlorine dioxide in situ. Chlorite and hypochlorite ion sources (e.g., sodium chlorite and sodium hypochlorite) are introduced into the well along with an acid. The well may then be flushed with water to force the reactants and chloride dioxide downwardly to the well formation.