The aim of this work was to determine some relevant rheological parameters of polymer/clay systems using mixing torque and mixing speed data obtained during processing in an internal mixer. The method used was originally proposed by Marquez et al. for monophase polymers, and is here applied to polymer/clay systems for the first time. Several clay-containing composites based on different polymer matrices (i.e., LDPE, HDPE, PA6, EVA) were used to verify the effectiveness of the method for measuring the flow curves of these polymer/clay systems. The results indicated that, for all the systems at low clay level, the rheological curves calculated with the Marquez method fit quite well those measured in both rotational and capillary rheometers. On the contrary, on increasing the amount of clay, the flow curves estimated by the melt mixing torque data still fit those measured by rotational rheometer while diverging from those measured by the capillary viscometer. Indeed, the polymer-clay systems do not obey the Cox-Merx rule at high filler content.