A process for preparing an aqueous solution comprising hypochlorous acid, the process comprising: (i) treating an aqueous starting solution comprising hypochlorite and a precipitable counter-ion, such as sodium carbonate, to form a counter-ion precipitate and a supernatant liquid having a pH greater than 6.0 (ii) removing at least part of said precipitate and subsequently (iii) acidifying at least part of the supernatant liquid with a strong inorganic acid other than hydrochloric acid, to a pH in the range of from 3.0 to 6.0, to form an aqueous product solution comprising hypochlorous acid. Preferably, the starting solution comprises a calcium hypochlorite, and the aqueous medium comprises mains water. The strong inorganic acid may be sulphuric acid, or phosphoric acid. A solution obtained by the process may be used as an antimicrobial or disinfectant, and may be stored for a period of at least three months. A process for modifying an aqueous hypochlorous acid solution comprises: (a) treating an aqueous solution comprising hypochlorous acid with lead nitrate to form a lead-based precipitate and (b) removing at least part of the lead-based precipitate to obtain a modified aqueous solution comprising hypochlorous acid.
