Residual mouth alcohol may be present if the deprivation period after the last drink has been less than 15 minutes. This is a typical situation for roadside testing or the use of personal screeners in restaurants and bars. As a result high false positive readings may occur evoking discussions about the usefulness of such devices. Most of the instruments based on the effect of IR-absorption are able to detect this kind of interference by continuously monitoring the slope of breath alcohol concentration versus time. This can not be done with electrochemical sensors as their time constant for diffusion and electrochemical reaction is in the range of several seconds. So if a breath sample containing a very high residual mouth alcohol concentration is applied to an electrochemical sensor it would result in such a false reading. The paper describes the new method of double sampling for detecting the presence of residual mouth alcohol. The technique is especially suited for breath testers using electrochemical sensors. Results from experiments with a wall-mounted breath tester applying this new technique are presented, underlining the usefulness in preventing false readings.