An in-line method was applied to determine local residence time distribution (RTD) at two places in a completely filled corotating twin screw extruder. Axial mixing effects of different types of elements were evaluated. Paddles +90 degrees induced flow patterns that appear to be circular, both upstream and downstream, whereas paddies -30 degrees induced flow patterns that appear to be circular and mainly upstream. Transport and single-lead elements induced backflow. The results could be explained from pressure differences and direction of drag flow in the elements. All elements were characterized using an equivalent additional mixing length lambda, dependent on the type of element and its position in the extruder, which may be practically useful for extrusion design. (c) 2005 Society of Plastics Engineers.