An experimental apparatus was built to measure the time constant of fine wet-bulb thermocouples in air. Rapid response solenoid valves (15 msec response time) were used to control airflow through tubing into which wet-bulb thermocouples were placed. Wet-bulb thermocouples (type T, 0.005 cm diameter) with the tip (bead) covered with a moistened wick were tested. Experiments were performed for air velocities ranging from 1.50 to 2.5 m sec(-1) (Reynolds number of 2,500 to 4,500) and wet-bulb temperature ranging from 12.5 to 17.4 degrees C. Experimental conditions were selected to simulate human respiratory conditions. Correlation between airflow velocity and wet-bulb thermocouple time constants was not significant at all levels.