Amorphous titanium phosphate film of a nominal composition 40TiO(2)-60P(2)O(5) (mol%) was prepared on a silica or slide glass substrate by a sol-gel method. Anomalous changes in protonic conduction were found in the films resulting from different heating temperatures. The electrical resistance of these films after appropriate heat treatments showed reversible dependence on relative humidity with fast response time and good chemical durability. The activation energy for proton transport in the film decreased with increasing relative humidity.