A mouthpiece system for deterring evasion of an alcohol breath test particularly a test wherein a breath sample is delivered without human supervision. The system includes a mouthpiece configured so as to render difficult the delivery of artificial breath samples through the use of hoses, balloons and the like. The mouthpiece has an oversize upper surface which includes a tapered lip around the mouthpiece inlet to receive the mouth of the user. The lip is surrounded by a wide apertured ring which, together with the lip, will not readily form an effective connection with a balloon or hose. An electrode on the mouthpiece is used for sensing and/or signalling the user. The impedance in contact with the electrode is sensed during testing to insure it falls within a range expected for a human user to deter use of evasion using filters or bogus gas delivery devices. Continuity of the impedance during testing is monitored to detect attempts to pass the mouthpiece to an accomplice. The electrodes are also connectable to a signal generator to cue the user to take a predetermined action at an appropriate time. The sensing and signalling functions may be time multiplexed or superposed to permit use of a single electrode to perform both functions.