Phase behavior at high temperatures and pressures have numerous applications, particularly in separation processes. The critical locus for various binary mixtures including non-polar, polar, and ionic species using grand canonical Monte Carlo simulations with histogram reweighting was deterined. The critical point is determined using the finite-size scaling approach of Bruce and Wilding. Different classes of the critical locus in binary mixtures, e.g., Class I and Class IIIb, are seen for systems including methane-ethane, methane-water, and water-NaCl. The choices for the intermolecular potentials and the combining rule between species have a significant impact on the critical locus. The effect of including polarization in the methane-water mixture on the critical locus is elucidated. This is an abstract of a paper presented at the AIChE Annual Meeting and Fall Showcase (Cincinnati, OH 10/30/2005-11/4/2005).