The first crystallographic study on an aromatic analogue of the antileukemic agent methylglyoxal bis(amidinohydrazone) (MGBG) is reported. Thus, the crystal and molecular structures of two different geometrical isomers of phenylglyoxal bis(amidinohydrazone) (PhGBG) sulphate were determined by single-crystal X-ray diffraction. Crystals were prepared by recrystallizing PhGBG sulphate using either water or aqueous ethanol (volume ratio ethanol:water 1:4) as the solvent. Depending on the solvent, different types of crystals were obtained although the PhGBG sulphate employed was in the both cases from the same synthesis batch that had been prepared according to classical methods from the corresponding glyoxal. When a crystal obtained from water was studied, PhGBG was found to exist solely in the form of the anti-anti isomer. e. the same isomer that has been observed in the cases of all mono- and dialkylglyoxal bis(amidinohydrazones) so far studied. However, when PhGBG sulphate was recrystallized from 20 % aqueous ethanol, the crystals obtained consisted of a different geometrical isomer. In this anti-syn isomer the carbon-nitrogen double bond closest to the phenyl group had the syn configuration. In the anti-syn isomer, there is an internal hydrogen bond between the two amidinohydrazone moieties, which may markedly contribute to the stabilization of the isomer. The anti-syn isomer of PhGBG is analogous to the only isomer of trifluoromethylglyoxal bis(amidinohydrazone) so far observed. PhGBG sulphate constitutes the first case in which two different geometrical isomers of a bis(amidinohydrazone) have been observed. In the case of the much studied aliphatic mono- and dialkylglyoxal analogues, isomerization of the bis(amidinohydrazone) backbone has never been observed. The structural flexibility of the bis(amidinohydrazone) chain of PhGBG is obviously attributable to the electron-withdrawing resonance effect and perhaps also to the inductive and hyperconjugative effects of the phenyl group. The obviously facile isomerization of PhGBG may markedly influence the biochemical properties of the compound.