A high‐pressure stopped‐flow apparatus was constructed which could be operative at hydrostatic pressures up to 3 kbar. Some preparative results on anionic &sgr;‐complex formation reactions are presented up to 1.5 kbar between 2,4,6‐trinitrobenzene (I) and hydroxide ion and between compound (I) and sulfite ion in water. It was found that this apparatus was sufficient to follow a chemical reaction whose half‐life was longer than several milliseconds.