The effect of humidity on the electrical conductivity in porous and dense ceramics of Ca12Al14-xSixO32Cl2+x (x=0, 1.7, 3.4) with a mayenite structure was examined at room temperature. Surface ionic conductivity was enhanced by increasing the humidity. The sample with x=3.4, which had a porosity of 30%, exhibited the largest conductivity change of about 4 orders of magnitude over the relative humidity range of 10-90% at 25 degrees C. The presence of Cl- ions on the surface most likely enhances the formation of hydronium ions which are responsible for proton transfer in physisorbed water layers.