Piezoelectric quartz crystals have been used for many years to detect particular odorants and other gases in air by specific modification of the crystal surface. Despite their potential for odorant monitoring they have not been widely commercialized. One of the reasons for this may be due to the variability of responses. The aim of these experiments was to investigate the sources of this variability. An automated odorant delivery system was developed that controls all the variables associated with the physical stimulus (concentration, flow rate, temperature and duration). Under these conditions, responses of individual crystals to an odorant were very repeatable but there was considerable variation between crystals, including cleaned blank crystals. It is concluded that even under similar conditions, the efficacy of surface modification varies substantially between crystals.