A humidity sensor using K2O-doped porous Ba0.5Sr0.5TiO3 ceramic is investigated. This ceramic humidity sensor exhibits a porous structure. The porous ceramic easily absorbs water vapour throughout the pores. The log-conductance against relative humidity (r.h.) sensitivity of this sensor is greater than 4 orders of magnitude in the range of 15∼95% r.h. at 400 Hz and 25°C. The adsorption process of the sensor is very fast. Its adsorption response time in r.h. variation from 15 to 95% is within a few seconds. Charging-discharging and complex impedance analysis techniques are used to analysis the direct current (d.c.) and alternating current (a.c.) response of this device under 50∼95% r.h. The sample can be polarized like electrolytes on charging process due to electrode space charge and grain surface water molecular polarization effects. The degree of polarization is enhanced with increasing r.h. The conduction carriers of this sensor in a humid atmosphere are ions and electrons, and the dominant conduction carrier is the ion. Using complex impedance analysis techniques, an equivalent circuit model associated with "non-Debye" capacitance is built. This model separates the sample into three regions: crystal grain, grain surface and electrode surface. The grain surface resistance and electrode surface resistance decrease sharply with increasing r.h., but crystal grain resistance is not affected by water vapour.