Palladium(II) co-ordination by the tertiary polyamines 2,5,8-trimethyl-2,5,8-triazanonane, 2,5,8,11-tetramethyl-2,5,8,11-tetraazadodecane, 2,5,9,12-tetramethyl-2,5,9,12-tetraazatridecane, 2,6,9,13-tetramethyl-2,6,9,13-tetraazatetradecane, 2,5,8,11,14-pentamethyl-2,5,8,11,14-pentaazapentadecane, 2,5,8,11,14,17-hexamethyl-2,5,8,11,14,17-hexaazaoctadecane and by the new compound 2,5,8,11,14,17,20,23-octamethyl-2,5,8,11,14,17,20,23-octaazatetracosane has been investigated by means of potentiometric (0.1 M NMe₄Cl aqueous solution, 298.1 K) and spectrophotometric measurements. The species present in solution and their stability constants have been determined. The first five amines form only mononuclear complexes, while the last two can give both mono- and bi-nuclear complexes in aqueous solution. The thermodynamic data show that the complexes with N-methylated polyamines are tremendously less stable than their non-methylated counterparts. The UV/VIS spectra of the complexes exhibit a marked red shift of the L→M charge-transfer bands with respect to the complexes of the non-methylated ligands. As a consequence of the reduced thermodynamic stability, both mono- and bi-nuclear complexes show a particular acid–base behavior with respect to those of the non-methylated amines. Such complexes can easily bind a proton or a hydroxide anion in aqueous solution, with consequent detachment of a co-ordinated nitrogen. The crystal structures of two binuclear complexes were determined. In both the metals show distorted square-planar arrangements of the donors.