A new Ru(II) complex (5) is synthesised from RuCl2(PPh3)(3) (3) and an N-heterocycliccarbene, 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene (4). Complex (5) catalyses the addition reaction of carboxylic acids to terminal alkynes very fast and quantitatively. The selectivity of the addition to phenylacetylene is dependent on the acidity of the added carboxylic acid. At decreasing pK(a), a change in regioselectivity from Markovnikov to anti-Markovnikov addition is observed. The intermolecular attack of acetic acid to aliphatic alkynes and intramolecular addition produces Markovnikov adducts. (C) 2002 Elsevier Science B.V. All rights reserved.