A precise experimental study of the reactive component of the complex impedance of small circular orifices has been made for a number of orifices varying in diameter from 2 cm down to 0.357 cm, with diameter/thickness ratios from 4 to 40, over the frequency range from 200 to 1000 c.p.s. The measurement of the impedance is performed with a precision impedance tube.A theoretical correction for the influence of the tube walls is applied by considering the orifice to act as a plane piston and taking into account the influence of all possible higher order modes of the tube in the neighborhood of the orifice. Comparison between calculated and measured values of reactance shows that, as far as this quantity is concerned, the assumption that the orifice acts like a plane piston appears valid for radii equal to or greater than 1 cm for “thin” orifices within the range of variables specified above. For orifices of radii less than 1 cm, however, a modification of the radius is necessary to make the classical theoretical equation for orifice reactance fit the measured data. The observations and analyses reported here are restricted to wave‐lengths much greater than the hole diameter and to the low velocity region where reactance is independent of particle velocity.