The buffers 4-morpholinepropanesulfonic acid (MOPS) and 3-morpholino-2-hydroxypropanesulfonic acid (MOPSO) are useful biological zwitterionic buffers within the pH range of 6.5 to 7.9 and 6.2 to 7.6, respectively. The solubilities of these buffers were determined in binary mixtures (1,4-dioxane + water) and (ethanol + water) at T =. 298.15 K by using the results of density measurements. It has been observed that MOPS induced liquid-liquid phase splitting for the mixtures of 40% to 90% (w/w) 1.4-dioxane in water. The two-liquid phase formation was visualized with disperse orange 25. The phase equilibrium boundaries, including the regions of one liquid. two liquids. (one liquid + one solid) and (two liquids + one solid), for the (MOPS + water + 1,4-dioxane) system have been determined experimentally at T = 298.15 K. The tie lines of the (liquid + liquid) equilibrium were also measured. The Othmer-Tobias and Bancroft equation were used to evaluate the reliability of the tie-line data. The binodal curve was fitted loan empirical equation and the effective excluded volume (EEV) model. The apparent free energies of transfer (Delta G(u)) of MOPS and MOPSO from water to 1.4-clioxane and ethanol solutions have been calculated from the solubility data. These Delta G(u) values were compared with those of some related biological buffers (TRIS, TAPS. TAPSO. and TABS). Furthermore, we also calculated the contribution of transfer free energies (Delta g(u)) of-OH group from water to 1,4-dioxane and ethanol solutions. (C) 2011 Elsevier Ltd. All rights reserved.