Applying Friction Stir Welding in an industrial environment requires knowledge about the ro-bustness of the welding process against imperfections in the joint line resulting from production devia-tions of the parts to be joined or from improper clamping devices. In this work the influence of gaps on the weld quality of friction stir welded butt joints of EN AW-5083-H111 is investigated in detail using the method Design of Experiments. As independent input variables the rotation speed, the feed rate, the tilt angle, a lateral tool offset and the gap width were chosen. The weld quality was quantified by its tensile strength. Moreover the fundamental response variables of the FSW process, such as weld temperature, axial force and spindle torque, were recorded and integrated in the empirical process model derived by regression analysis. It was found that each response variable decreases with increasing gap width, whereas the largest impact was identified for the axial force and the temperature at the weld surface. In order to improve the joint strength for a given gap width, raising the rotation speed and the tilt angle or lowering the feed rate, respectively, proved to be suitable. However, interactions between some process parameters have to be considered.