The aim of this paper is to highlight the problems that arise when the immunity of measurement instruments to power line voltage disturbances has to be evaluated. The lack of an international product standard puts at the user's disposal only some basic standards which, in giving only general criteria for test result classification, make it difficult to choose suitable acceptance criteria and, consequently, to plan a proper set of tests, especially for very complex instruments. After an introduction on the state of art of electromagnetic compatibility (EMC) international standards, a measurement technique adopted by the authors to evaluate the immunity of complex instruments to conducted disturbances is presented. Finally, experimental tests carried out on VXI instruments are given and discussed exhaustively.