Twelve male college-age subjects and a thermal rnannequin were studied for their thermal, physiological, and subjective responses to.humidity step-changes. Using twin climate chambers, subjects spent half of the three-hour experiment in 20% RH and the remainder in 80% RH, while the operative temperature was held constant. Both humidity up-steps and down-steps were investigated. Subjects and the rnannequin were naked, or wore 1~0 clo woolen or polyester ensembles. Mannequin measurements indicated that between 37% and 42% of the woolen ensernble’s thermodynamic response to humidity step-changes affected the sensible heat balance of the wearer; while the thermodynamic response of the polyester ensemble was negligible~ Human subjects’ mean skin temperatures indicated an effect of the humidity step-changes in the wool, polyester, and naked cases, but the effect was greatest while wearing wool. Thermal sensation responses to the humidity step-changes were greatest while the subjects were wearing wool, but there were significant short-term effects after the naked and polyester humidity down-steps, suggesting a thermodynamic response in human skin tissue to hurnidity transients. Generally the humidity down-steps produced a bigger impact on thermal sensations than the opposite humidity up-steps.