The infrared spectra of Na and Ca complexes of mefenamic acid were investigated in the region between 4000 and 400 cm(-1). These spectra were compared with X-ray powder diffraction patterns of complexes. It is shown that the proposed structure for these complexes obtained from the infrared spectra was supported by X-ray powder diffraction measurements. Bands due to v(as)(COO)(-) and v(s)(COO)(-) stretching vibrations appear at about 1580 cm(-1) and in the range 1390-1400 cm (-1) in the infrared spectra of the samples, respectively. The values of NH deformation vibrations do not change in the spectra of mefenamic acid and its metal complexes. On the other hand, molecular mechanic calculations and conformational analysis of three compounds were also established in the present work. As a result of these theoretical studies, the atomic planes and the peak assignments of the powder diffraction patterns were determined. As a result of these experimental and theoretical investigations, it may be concluded that metal atoms are connected to the carboxyl group of the mefenamic acid. (C) 1999 Elsevier Science B.V. All rights reserved.