In this study, the adsorption and thermodynamic parameters of Diazepam drugs adsorbed onto six activated carbons were determined. A simulated gastric fluid was applied at pH 1.2 for 4 h. The samples were characterized by FTIR, N2 isotherm at 77 K, and CO2 at 273 K. The experimental adsorption was fitted by isotherm models: Langmuir Type I and II, DR, Halsey, Freundlich, Harkins-Jura, Temkin, and BET. UV visible spectra monitored residual drugs. The results showed relationships between temperature increase, adsorbent characteristics, and the behavior of these drugs in acid solution. The positive values of all the isosteric adsorption enthalpies determined from the slope Van’t Hoff plot (R 2 >97) indicated the endothermic nature of the adsorption process. In all cases, ΔG<0 so that the spontaneous character of the adsorption process was attested. Moreover, the positive values of ΔS stated that the randomness increased at the solid-solution interface during the adsorption process.