As much as 60 million liters of lubricating fluids end up in the environment annually, and thus, the environmental impact of lubricants is increasingly recognized in addition to meeting the rheological and tribological requirements. Although US Environmental Protection Agency-approved environmentally acceptable base fluids are available, there is a lack of additives that are both nontoxic and effective in wear protection. This study reports the successful development of a new class of ionic liquids (ILs) with demonstrated significantly lower aquatic toxicity and superior friction reduction and wear protection capabilities compared with a commercial lubricant additive and some ILs reported in the literature. Specifically, ammonium phosphate and phosphonium phosphate ILs with four-carbon alkyls have been identified with balanced oil solubility, thermal stability, toxicity, and lubricity, which provide fundamental insights for future development of eco-friendly ILs.Development of eco-friendly ionic liquids as friction reduction and wear protection lubricant additives for improved energy efficiency and reduced environmental impact.