Under 254 nm irradiation in chloroform, [Re2Cl8]2− is converted cleanly to [Re2Cl9]−. The reaction is initiated primarily through light absorbed by the chloroform. The experimental rate expression is View the MathML source, where I0 is the incident light intensity and fS is the fraction of light absorbed by the solvent. The rate law is consistent with mechanisms involving either [Re2Cl8]− or [Re2Cl9]2− as an intermediate.