Ketoconazole (KET) is an imidazole derivative antifungal drug with very low aqueous solubility and bioavailability, widely used in a variety of formulations for oral and topical administration. Based on KET co-crystallization potential with dicarboxylic acids a Ketoconazole – Fumaric Acid (KET-FUM) co-crystal was obtained [1]. The solubility of this co-crystal is 100 times higher than pure KET, so it can be considered a good candidate for oral administration in lower doses for the fungal infections treatment. Therefore, compatibility studies with the usual pharmaceutical excipients of this co-crystal are needed to be performed. Physical mixtures of KET-FUM with corn starch, microcrystalline cellulose, magnesium stearate, polyvinylpyrrolidone, hydroxypropyl methylcellulose, colloidal silica and talc, in 1:1 ratio, were prepared and analysed by powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The PXRD and FTIR analysis did not reveal interactions between co-crystal and the tested excipients. The DSC analysis shows no interactions between KET-FUM and corn starch, microcrystalline cellulose, hydroxypropyl methylcellulose, colloidal silica, talc and polyvinylpyrrolidone, for the first five excipients the values of thermal events remaining practically unchanged, and for the last one a melting peak appears at a slightly lower temperature than in pure co-crystal (168°C). The mixture of KET-FUM with magnesium stearate exhibits a totally different thermal behavior than pure components [2]. During the heating process four endothermic events were observed: the first between 50-80°C corresponds to the dehydration of sample, and the next three, with maxima at 94, 108 and 124°C respectively, are probable due to the physical-chemical interactions that appear between co-crystal and magnesium stearate. Also, the decomposition of the sample occurs at lower temperature than the melting of the pure co-crystal. Due to this interaction the magnesium stearate should be avoided as tablet lubricant in KET-FUM formulations