A microsensor of cylindrical geometry, 0.015 cm diam or less, has been developed for measuring changes in relative humidity in a gas stream. The sensor works on the principle of impedance variations in a thin dielectric film with changes of humidity of the surrounding gas. The cylindrical shell of dielectric material on which the sensor depends is aluminum oxide. The sensor was found to be sensitive, in terms of either resistance or capacitance, to changes in relative humidity in the gas for relative humidities greater than 30%. The very small size of the humidity probe, combined with its relatively fast response time, makes it useful for measuring gradients in humidity near moist surfaces, or for measuring concentrations of water vapor in turbulent media like the earth's atmosphere.