Near-critical water (NCW) has drawn increasing attention as an environmentally benign medium for organic chemical reactions, especially acid/base catalysis without added acid or base. However, problems such as low reaction rates and poor selectivity exist for organic syntheses in NCW that greatly hinder the application of this attractive technique in industrially interesting processes. We have developed a novel solvent system to accelerate base-catalyzed reactions by the addition of NH3 to NCW. The reactions demonstrated include hydrolysis of cinnamaldehyde and aldol condensation of benzaldehyde with acetaldehyde. The reaction kinetics of the two reactions were determined. The effects of the NH3 concentration and temperature on the reactions were also investigated. The pOH of NH3-enriched NCW was estimated. The results demonstrate the feasibility of using NH3-enriched NCW as a sustainable reaction medium for the acceleration of base-catalyzed reactions.