Two cases are presented which highlight the potential for abused volatile substances to interfere with the results of evidential breath testing equipment. Home Office guidance, issued in 1983, advises that breath testing devices should not be used when a driver is suspected of glue-sniffling, but the advice does not appear to be widely known. The implications for both police officers and police surgeons are discussed. New evidential breath testing devices, which are planned to be introduced into the UK in the first half of 1997, are designed to recognize interfering substances such as solvents. The potential problems these new devices may pose are considered.