In this paper the results of a validation study of the Dutch and Norwegian versions of the Illegal Aliens (IA) Scale are discussed. The main objective is to evaluate the usefulness of the ThreeStep TestInterview (TSTI) as a pretesting method in a validation study. The TSTI is a new technique specifically developed to detect problems with selfcompletion questionnaires. The studys aims were twofold: (1) to evaluate the usefulness of the TSTI for detecting problems regarding attitude measurement; and (2) to identify potential problems with the IA Scale. The IA Scale is a Likerttype attitude scale consisting of 20 parallel interval items. Its main purpose is for large comparative sample studies of political and ideological attitudes, for example between subgroups within populations or between countries. The TSTI procedure appeared to be very productive in terms of the number and kind of problems that were found regarding the Dutch and Norwegian versions of the IA Scale. It is concluded that the TSTI is useful as a diagnostic tool in validation studies of instruments that are aimed at measuring attitudes.