The anti-Markovnikov hydration of terminal alkynes to give aldehydes is catalyzed by complexes derived in situ from air-stable [CpRu(eta6-naphthalene)]PF6 (C) and 6-aryl-2-diphenylphosphinopyridines (L). Ligands L are readily available from a modular synthesis. Increasing the size of the ligand C-6 aryl group in the order R = Ph < mesityl < 2,4,6-triisopropylphenyl < (2,4,6-triphenyl)phenyl gave hydration catalysts of highest known activity. [reaction: see text]