Purpose: Telemetric temperature capsule systems are wireless, relatively non-invasive and easily applicable in field conditions, and have therefore great advantages for monitoring core body temperature. However, the accuracy and responsiveness of available capsule systems have not been compared previously. Therefore, the aim of this study was to examine the validity, reliability and inertia characteristics of four ingestible temperature capsule systems (i.e. CorTemp, e-Celsius, myTemp and VitalSense). Methods: Ten temperature capsules were examined for each system in a temperature controlled water bath during three trials. The water bath temperature gradually increased from 33°C to 44°C during Trial 1 and 2 to assess the validity and reliability, and from 36°C to 42°C in Trial 3 to assess the inertia characteristics of the temperature capsules. Results: A systematic difference between capsule and water bath temperature was found for CorTemp (0.077°C±0.040°C), e-Celsius (-0.081°C±0.055°C), myTemp (-0.003°C±0.006°C) and VitalSense (-0.017°C±0.023°C) (p<0.010), with the lowest bias for the myTemp system (p<0.001). A systematic difference was found between Trial 1 and Trial 2 for CorTemp (0.017°C±0.083°C, p=0.030) and e-Celsius (-0.007°C±0.033°C, p=0.019), whereas temperature values of myTemp (0.001°C±0.008°C) and VitalSense (0.002°C±0.014°C) did not differ (p>0.05). Comparable inertia characteristics were found for CorTemp (25±4 sec), e-Celsius (21±13 sec) and myTemp (19±2 sec), while the VitalSense system responded more slowly (39±6 sec) to changes in water bath temperature (p<0.001). Conclusion: Although differences in temperature and inertia were observed between capsule systems, an excellent validity, test-retest reliability, and inertia was found for each system between 36°C and 44°C after removal of outliers.