An on-line capillary rheometer concept using an in-house built sampling and testing device, as well as a commercially available portable rotational rheometer, is proposed. Off-line validation was carried out by checking the behaviour of a standard silicone oil. On-line validation was provided through the comparison of the viscosity of two commercial polymer melts with that obtained via conventional off-line experiments. The apparatus allows the quick measurement of the evolution of the rheology of polymer systems along the length of an extruder. The ability to characterise rheologically reactive systems is illustrated by monitoring the peroxide induced degradation of polypropylene along a twin screw extruder.