Novel impedance-type humidity sensors were made of 1-(4-carboxylic acid phenyl)-2,5-dimethyl-1H-pyrrole that was doped with KOH and K2CO3 salts. The humidity-sensing properties of 1-(4-carboxylic acid phenyl)-2,5-dimethyl-1H-pyrrole were improved by doping with two alkali salts (KOH and K2CO3), whose dissociation constants differed greatly from each other. The electrical properties of 1-(4-carboxylic acid phenyl)-2,5-dimethyl-1H-pyrrole that was doped with KOH or a mixture of KOH and K2CO3 were examined in detail and found to be functions of relative humidity (RH). Additionally, the effects of the salts on the sensing properties (linearity and sensitivity) were elucidated. The sensor that was made of the 1-(4-carboxylic acid phenyl)-2,5-dimethyl-1H-pyrrole/KOH/K2CO3 composite film exhibited favorable impedance, high sensitivity, good linearity, small hysteresis, a short response/recovery time, a weak dependence on temperature and high long-term stability over the humidity range studied. The sensing mechanism of the 1-(4-carboxylic acid phenyl)-2,5-dimethyl-1H-pyrrole/KOH/K2CO3 composite film was elucidated using impedance plots.