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International dosimetry protocols require measurements made with a vented ionization chamber to be corrected for the influence of air density by using the standard temperature-pressure correction factor. The effect of humidity, on the other hand, is generally ignored with the provision that the relative humidity (RH) is within certain limits (typically 20% to 80%). However, there is little experimental data in the published literature as to the true effect of humidity on modern reference-class ionization chambers. This investigation used two different radiation beams – a Co-60 irradiator and an Sr-90 check source – to examine the effect of humidity on several Farmer-type ionization chambers.An environmental cabinet controlled the humidity. For the Co-60 beam, the irradiation was external, whereas for the Sr-90 measurements, the source itself was placed within the cabinet. Extensive measurements were carried out to ensure that the experimental setup provided reproducible readings. Four chamber types were investigated: IBA FC65-G, IBA FC65-P, PTW 30013 and Exradin A19. The different wall materials provided potentially different mechanical responses (i.e., in terms of expansion/contraction) to the water content in the air. The relative humidity was varied between 8% and 98% and measurements were made with both increasing and decreasing humidity to investigate any possible hysteresis effects.Measurements in Co-60 were consistent with the published data obtained with primary standard cavity chambers in ICRU Report 31 (i.e., a very small variation <0.1% for RH >10%). The measurements in the Sr-90 field showed no dependence with the relative humidity, within the measurement uncertainties (0.05%, k = 1). Very good repeatability of the ionization current was obtained over successive wet/dry cycles, no hysteresis was observed, and there was no dependence on chamber type.These results indicate that humidity has no significant effect on these particular types of ionization chambers, consistent with recommendations in current megavoltage dosimetry protocols.