Critical properties of the NaCI-H₂0 fluid system are of fundamental interest to a variety of geochemical applications including fluid inclusion studies, numerical modeling of hydrothermal systems, and development of theoretical models for two·component fluid systems. Although many workers have expressed interest in NaCl·H₂0 fluid critical properties, most studies have been limited to small compositional ranges with little agreement among data sets at higher salinities. Critical densities are recorded in only one of these reports, and no studies have determined the locations of NaCl-H₂0 critical isochores (PT projections of critical densities). Furthermore, no studies to date have determined critical properties of NaCl·H₂0 solutions in excess of room temperature saturation (26.4 wt.% NaCl). / Master of Science