The determination of stability constant of Rifampic in complexes with rare earths metal ions in temperature range 303-318 K at constant ionic stren gth 0.1M.in 50% dioxane-water mixture by adopting pH metric technique been studied. The ther modynamic parameters ( ∆ G,
∆ H and ∆ S) were calculated from values of stability constant at dif ferent temperatures. The formations of metal complexes were found to be spontaneous and exotherm ic nature.