Excess K+ ions in K+-beta-ferrite (K1.33Fe11O17) have been deintercalated by chemical oxidation using iodine, and humidity-sensitive characteristics have been measured for K+-beta-ferrites containing different contents of K+ ions. Corresponding to the extent of deintercalation, the impedance change with a change in the relative humidity got smaller. The humidity-sensitive characteristics almost disappeared at the fully deintercalated composition of KFe11O17, accompanied by a degradation of the response characteristics. The humidity-sensitive characteristics were independent of the amounts of absorbed water. It was deduced that when water vapour was absorbed on the original K+-beta-ferrite, the excess K+ ions seeped out from the alkali layer to the adsorbed water to increase the ionic conduction due to K+ and OH- ions. This model was discussed on the basis of the effects of co-adsorption of weak acids.