Pyrite is a chemically active mineral, the surface of which oxidizes when exposed to the atmosphere and to aqueous environments during mining and mineral processing operations. As a result, a hydrophilic surface state is created which impacts collector adsorption and flotation response. In this regard, the collectorless flotation of oxidized pyrite was studied by ultrasonic pretreatment conditioning and flotation in CO2 saturated water. The oxidized pyrite was ultrasonically pretreated (5-60 min) and micro-flotation tests were conducted under different pH conditions (pH 3-10). Furthermore, the effect of CO2 saturated water on collectorless flotation recovery of oxidized pyrite was also investigated at different saturation temperatures (4-33 degrees C). According to the results, ultrasonic pretreatment had a significant effect on collectorless flotation recovery of oxidized pyrite with recovery values of 50% after 60 min of sonication. In CO2 saturated water, the flotation recovery of ultrasonically treated oxidized pyrite at room temperature gradually increased from 50% after 15 min of sonication to over 65% after 60 min of sonication. Further, saturation of DI water with CO2 at 4-7 degrees C promoted both flotation kinetics and flotation recovery, resulting in over 90% flotation recovery in less than 30 s after 15 min of sonication.