Abstract: Symmetrical or unsymmetrical dialkyl and dicycloalkyl ethers, or alkyl-cycloalkyl (polycycloalkyl) ethers containing a total of 2 to 24 carbon atoms in which the oxygen atom is attached to primary or secondary, but not to tertiary carbon atoms are cleaner burning and cetane enhancing supplements for diesel fuels. The ether supplements are added in an amount of about 0.5 to 5% v/v. Cetane numbers can be further enhanced by adding 0.05 to 0.5% by weight of an alkyl or dialkyl peroxide compound having one to 12 carbon atoms in the alkyl group.