The (±)-benzyl(methyl)(4-methylphenyl)(naphthalen-1-yl)arsonium cation has been synthesized and subsequently resolved by fractional crystallization of monohydrogen [R-(R*, R*)]-2, 3-bis(benzoy1oxy)-butanedioate salts. The separated diastereoisomers were converted into the corresponding optically active arsonium bromides by ion-exchange column chromatography. The absolute configuration of the arsonium cation exhibiting a positive rotation at 589 nm (sodium D line) has been established as (R) by single-crystal X-ray analysis of both the bromide and hexafluorophosphate salts. The arsonium bromide with [α] D + 54.8° (c, 0.62 in CH2Cl2) crystallizes in the orthorhombic space group P212121 (D24, NO. 19) with a 22.472(8), b 15.724(7), c 12.585(5) Å and U 4447(3) Å3. The corresponding hexafluorophosphate with [α]D +19.3° (c, 0.5 in CH2Cl2) crystallizes in the same space group with a 23.56(2), b 16.40(1), c 13.12(1) Å and U 5067(6) Å3. Benzylidene transfer to benzaldehyde from the arsonium ylide derived from either of the arsonium salts produced optically pure (-)-(S)-methyl(4-methylphenyl)(naphthalen-1-yl)arsine, [α]d -115.9° (c, 0.593 in CHCl3), and partly resolved [R-(R*, R*)]-2, 3-diphenyloxiran.