The Wyatt Optilab rEX is configured as a standalone differential refractometer, used principally to measure the specific refractive index increment (dn/dc) of polymer solutions; dn/dc is a required parameter for analysis of data from the light scattering detectors on our two GPCs. Ordinarily, the Optilab rEX is configured with a 658 nm fiber-optic laser diode source, but we also have sources operating at 927 nm and at 685 nm, which allow for an assessment of the wavelength dependence of dn/dc. The Optilab is equipped with Peltier units for temperature control and for the measurement of dn/dc at temperatures above and below ambient.