Tri(allyl)- and tri(methylallyl)arsine complexes of palladium(II) and platinum(II) with the formulae [MX₂L₂] (M = Pd, Pt and X = Cl, Br, I): [Pd₂Cl₂(μ-Cl)₂L₂], [PdCl(S₂CNEt₂)L] and [Pd₂Cl₂(μ-dmpz)₂L₂] [L = As(CH₂CHCH₂)₃ (L′), As(CH₂CMeCH₂)₃ (L″), dmpz = 3,5-dimethylpyrazolate] have been prepared. All the complexes have been characterized by elemental analyses and by IR and NMR (¹H, ¹³C, ¹⁹⁵Pt) spectroscopy. The stereochemistry of the complexes has been deduced from the spectroscopic data. The molecular structures of complexes [MX₂{As(CH₂CHCH₂)₃}₂] (M = Pd, X = Cl or Br), [MX₂{As(CH₂CMeCH₂)₃}₂] (M = Pd, X = Cl or Br; M = Pt, X = Cl) and [Pd₂Cl₂(μ-Cl)₂{As(CH₂CMeCH₂)₃}₂] have been established by single crystal X-ray diffraction analyses. The mononuclear complexes exclusively adopt the trans configuration with the exception of [PdCl₂L″₂], which could be isolated as cis and trans isomers. In the binuclear derivative the arsine ligands are attached to an envelope-shaped Pd–(μ-Cl)₂–Pd rectangle with a trans (anti) orientation towards each other. The mononuclear complexes are slightly photoreactive upon irradiation in their long-wavelength absorption band.