An in-line rheometer previously described was evaluated for use in the upgrading of post-consumer, recycled HDPE. Feed resin of variable melt index was brought to a preset, uniform viscosity by peroxide induced crosslinking in a twin-screw extruder. Closed loop control was demonstrated using the rheometer as a viscosity sensor. Set-point tracking was achieved using PID-IMC, Dahlin and minimum variance (MV) controllers. The Dahlin controller showed the best load rejection.