Solid-state interactions between trimethoprim and the esters of 4-hydroxybenzoic acid (parahydroxybenzoates or parabens) used for anti-microbial preservation are investigated. The formation of a crystalline 1/1 molecular compound between trimethoprim and methyl parahydroxybenzoate is demonstrated by differential scanning calorimetry, X-ray powder diffraction, Fourier transform infrared spectroscopy, and solid-state C-13-NMR. Interactions between trimethoprim and 4-hydroxybenzoic acid and its ethyl,propyl and butyl esters were not observed. The nature of the trimethoprim and methyl parahydroxybenzoate intermolecular bonding is discussed briefly.