The tetrathioperrhenate anion, ReS4- (1), is shown to form adducts with a variety of alkenes. The alkene adducts form reversibly, and those of norbornene and norbornadiene were isolated. For norbornene, the K-eq's range from 12 000 (CH2Cl2 solution) to 2650 M-1 (pyridine solution). The following values were determined (MeCN soln): Delta H = -53 kJ mol(-1) and BS = -98 J.mol(-1).K-1. The solid-state structures of adducts derived from norbornene and norbornadiene, Ph4P[ReS2(S2C7H10)] (2) and (Ph4P)(2)[(ReS2)(2)(S4C7H8)], respectively, reveal tetrahedral Re centers wherein the Re-SR bonds are elongated by 0.1 Angstrom compared to those in 1. Additionally, the RS-Re-SR angle is contracted to 86.7 degrees, reflecting the constraint of the chelate backbone. Electrochemical measurements show that the reduction of 2 induces dissociation of norbornene from ReS42-. Ethylene binds to 1 only weakly at room temperature, but this adduct is directly observable by low-temperature UV/vis spectroscopy. The adduct of dimethyl maleate and 1 could only be observed in the neat alkene; 1 does not catalyze the isomerization of dimethyl maleate to dimethyl fumarate.