To explain the differences obtained when sampling 4,4'-diphenylmethane diisocyanate (MDI) in industrial atmospheres, with solvent and solvent-free sampling techniques, as previously described, a complex "MDI matrix" was looked for. The dimer of MDI and MDI treated with water (compound X) were evaluated as possible candidates for the "complex MDI matrix". The compound X was analyzed by high pressure liquid chromatography, infrared spectrometry, mass spectrometry, nuclear magnetic resonance, and differential scanning calorimetry. The melting point was also determined. Suggested from these experiments is that MDI forms a hydrate with water and that this hydrate might be the explanation for the ambiguous results obtained when sampling MDI by different techniques in the industrial atmosphere.