The material Ammonium heptamolybdate tetrahydrate has been shown to be an ionic conductor with t(ion) similar to 0.97. Its conductivity is strongly humidity dependent dominated by surface ion transport. This effect has been utilized to develop a humidity sensor. A linear change of similar to 2000 times in the resistance of the sensor with ’finger-type’ electrodes has been observed when relative humidity changes from 10% to 95%. The response time has been found to be similar to 2 min when humidity changes from low to high R.H. values. The results have also been compared with a conventional disc-type sensor having electrodes deposited on both sides.