The volume of nylon‐walled ionization chambers varies with relative air humidity due to the hygroscopic properties of nylon. A 6% increase in exposure calibration factor of a commonly employed 0.6 cm3 nylon‐walled ionization chamber was observed when the relative air humidity decreased from 98% to 11%. The increase as well as the decrease in volume shows an initial fast change during the first day followed by an exponential change with half‐lives of about 1.2 days and 2.4 days, for water uptake and water loss, respectively.