Dicationic platinum alkyne complexes were generated in situ by substitution of ethylene in [Pt(PNP)(C 2 H 4 )]-(BF 4 ) 2 (PNP = 2,6-bis(diphenylphosphinomethyl)pyridine) with alkynes at low temperature. The dicationic acetylene complex readily adds water to form the platina-acetaldehyde complex [Pt(PNP)(CH 2 CHO)]BF 4 , which was analyzed by X-ray diffraction. 1 H and 31 P NMR studies were performed to elucidate the mechanism of formation of [Pt(PNP)(CH 2 CHO)]BF 4 . A reversible acid−base equilibrium between the platina-acetaldehyde and the corresponding η 2 -vinyl alcohol complex [Pt(PNP)(CH 2  CHOH)] 2+ was observed. The complexes with terminal alkynes (propyne and 1-hexyne) gave with water a mixture of Markovnikov and anti-Markovnikov addition products [Pt(PNP){CH 2 C(O)R 1 }]BF 4 and [Pt(PNP){C(O)CH 2 R 1 }]BF 4 (R 1 = Me, n-Bu) in a ratio of 1:4. However, with tert -butyl-and phenylacetylene C−H bond activation occurred, yielding the σ-alkynyl complexes [Pt(PNP)(CCR 2 )]BF 4 (R 2 = t-Bu, Ph). Complexes with internal alkynes R 3 CCR 4 (R 3 = Me; R 4 = Me, n-Pr) react with water and form the corresponding β-ketonyl complexes [Pt(PNP){CHR 3 C(O)R 4 }]BF 4 . Moderate regioselectivity was observed for 2-hexyne.