Ionization chambers often exhibit a stem effect, caused by interactions of radiation with air near the chamber end, or with dielectric in the chamber stem or cable. These interactions contribute to the apparent measured exposure. To determine the stem effect for several common ionization chamber systems, exposures were measured with TLD capsules placed at the center of60Co fields of various sizes. These exposure measurements then were repeated with various ionization chamber systems, including two Victoreen R meters (25‐ and 100‐R chambers), a Capintec 192 dosimeter with a Farmer 0.6‐cm3probe, a PTW transit dose probe, and an EG&G IC‐18 probe with a Keithley 610‐B electrometer. From a comparison of TLD and ionization chamber measurements of the variation in exposure rate with field size, stem corrections for the different systems were determined within 1%.