Exposure to benzene-1,2-diol (pyrocatechol), a common soil contaminant, has been shown to cause loss of strength in concrete. Synthesis and characterisation of two compounds formed when Portland cement comes in contact with benzene-1,2-diol has been conducted. These compounds are (benzene-1,2-diolato(1-))hydroxidocalcium(II) (formula: [Ca(C6O2H5)OH]) and triaqua(benzene-1,2-diol)(benzene-1,2-diolato(1-))hydroxidocalcium(II) (formula: [Ca(C6O2H5)(C6O2H6)(H2O)(3)(OH)]). The compounds may play a role in the deterioration process. Characterisation was conducted in terms of thermal decomposition behaviour, infra-red spectra and crystal structure (using powder X-ray diffraction). The likely thermal decomposition reactions of both compounds have been determined, and features in the infra-red spectra assigned to molecular vibrations. Both structures have been solved, with the exception of the location of some hydrogen atoms, and the structures refined using Rietveld refinement methods. It is anticipated that both the thermal analysis data obtained, and crystal structures deduced, in this article can now be used to quantify these phases in cement and concrete exposed to benzene-1,2-diol.