A new class of stable aromatic fluids structurally related to the polyphenyl ethers are described. Designated as aromatic C-ethers, they are characterized by resistance to degradation by oxidation, heat, ionizing radiation, and hydrolysis. They are oxidatively stable in the temperature range of 500–600 F and have thermal decomposition temperatures above 700 F. Acceptable damage levels are observed at radiation dosage rates up to 107 rads at 375 F. The C-ethers offer considerable promise as high-temperature lubricants with outstanding gear load-carrying capacities. The fluids are compatible with many elastomers and most engineering materials and are essentially nontoxic.