Copper(I) catalysed azide–alkyne cycloaddition reactions (CuAAC) occur smoothly in liquid ammonia (LNH3) at room temperature to give exclusively 1,4-substituted 1,2,3-triazoles with excellent yields (up to 99%). The CuAAC reactions in liquid ammonia require relatively small amounts of copper(I) catalyst (0.5 mole%) compared with that in conventional solvents. The product can be obtained conveniently by simply evaporation of ammonia, indicating its potential application in industry. The rate of the CuAAC reaction in liquid ammonia shows a second order dependence on the copper(I) concentration and the reaction occurs only with terminal alkynes. Deuterium exchange experiments with phenyl acetylene-d1 show that the acidity of the alkyne is increased at least 1000-fold with catalytic amounts of copper(I) in liquid ammonia. The mechanism of the CuAAC reaction in liquid ammonia is discussed.