The aim of this study was to evaluate the accuracy of 3 thermosensitive microsensors that record "wear time" of removable oral appliances (OAs). For in vitro testing, TheraMon (sensor T, n = 20), AIR-AID SLEEP (sensor A, n = 30) and DentiTrac (sensor D, n = 16) microsensors were placed in a water bath to simulate long and short durations of wear. Their accuracy was also assessed when they were embedded in 3 materials: acrylic, polyvinylchloride and thermoactive acrylic. In vivo testing included 14 volunteers, who wore maxillary retainers embedded with sensors A and D for 30 nights. Logs of appliance use were compared with readouts from the sensors. In the in vitro long duration of wear assessment, sensor A, with a mean absolute response difference (MARD) of 1.67 min. (SD 1.41), was significantly more accurate than sensor T (MARD 3.53 min., SD 9.80) and sensor D (MARD 4.48 min., SD 8.46). For short duration of wear, sensor A (MARD 1.41 min., SD 3.60) and sensor T (MARD 1.68 min., SD 7.64) were equal in accuracy and significantly better than sensor D (MARD 14.07 min., SD 10.20). The embedding material had no effect on the recording accuracies of the microsensors. In vivo, there was no significant difference between sensors A and D. All 3 microsensors are highly accurate and reliable and can be used to record wear time of a removable OAs fabricated from various materials.