The didentate ligand 2-phenylazopyridine (azpy) can – in theory – give rise to five different isomeric complexes of the type [Ru(azpy)₂Cl₂], of which three have been known since 1980. The molecular structures of the cis-dichlorobis(2-phenylazopyridine) ruthenium(II) complexes α-[Ru(azpy)₂Cl₂] and β-[Ru(azpy)₂Cl₂] (in which the coordinating pyridine nitrogen atoms are in mutually trans and cis positions, respectively, whilst the azo nitrogen atoms are in mutually cis positions) were unambiguously determined in the early 1980s. The third isomer, γ-[Ru(azpy)₂Cl₂], has for two decades, erroneously, been assumed to be the all-trans isomer. In a recent communication we have proven that for this γ isomer the chloride ions are indeed in a trans geometry, but the pyridine nitrogen and azo nitrogen atoms of the two azpy ligands are in mutually cis geometries. In this paper the isolation of a fourth isomer is presented, the hitherto unknown δ-[Ru(azpy)₂Cl₂]. The isomeric structure of δ-[Ru(azpy)₂Cl₂] has been determined by ¹H-NMR spectroscopy and single-crystal X-ray diffraction analysis, and is the all-trans isomer. The bis(azpy)–ruthenium(II) isomers are of interest because of the pronounced cytotoxicity they exhibit against tumour cell lines and could be very useful in the search for structure–activity relationships of antitumour-active ruthenium complexes, as among the isomers there is a significant difference in activity. It is of paramount importance to have a good understanding of the structural and spectroscopic properties of these complexes, which in this paper are compared and discussed, with a particular emphasis on 1D and 2D ¹H NMR spectroscopies.