New methods are needed to assess microenvironmental and personal exposures to ozone. A real-time ozone sensor using a piezoelectric quartz crystal has been developed and evaluated. The crystal is coated with polybutadiene which reacts irreversibly with ozone, resulting in a mass increase on the surface of the crystal which, in turn, alters its natural oscillation frequency. The rate of change in frequency is proportional to the concentration of ozone and is recorded by a datalogger with a time resolution of 10 min. Experiments were conducted to investigate sensor response, useful lifetime, and potential interferences. Monitor components such as a pump, power supply, and datalogger were arranged to fit within a compact enclosure. Five prototype ozone monitors were assembled and evaluated in the laboratory and in a field study of two offices and two residences in Southern California. Field measurements made with two quartz crystal-based ozone monitors exhibited precision ranging from 6 to 27% (13% on average) and bias ranging from -37 to 2% (-13% on average) when compared to a reference method (UV photometric detection) for real-time ozone measurement. The monitor has a sensitivity of 3 ppb ozone (limit of detection for S/N = 3) and a useful lifetime of ~20 h at an average ozone concentration of 50 ppb (1000 ppb-h). This prototype monitor is smaller, lighter, and more affordable than existing UV ozone analyzers and with further development may be suitable for real-time personal ozone monitoring.