A mathematical model has been derived to describe the pressure drop along the conveying and reverse sections of a partly full, fully intermeshing co-rotating twin-screw extruder. The model extends previous work on single screw extrusion, whereby the Benbow-Bridgwater model of paste rheology is combined with a force balance on a plug of paste. Experiments to verify the model have been carried out using a laboratory scale twin-screw extruder fitted with a hydraulic feeder and a modified barrel section allowing the collection of pressure data at numerous points along the extruder. Initial results suggested that the model was able to correctly predict the trends observed but underestimated the absolute values of the pressure. This was attributed to a change in the rheology of the paste due to viscous heating effects. Further experimental work was carried out to determine the relationship between the rheological parameters and temperature; this allowed a temperature correction to be made to the initial data after which the agreement with the predictions was greatly improved.