Impedance-type humidity sensors were fabricated from WO3 particles to which had been added KOH or Au nanoparticles (Au NPs). The humidity-sensing (sensitivity and linearity) and electrical properties of the WO3 were improved. Adsorption dynamic analysis and molecular mechanical calculations revealed that the sensitivity of KOH-added WO3 exceeded that of Au NPs-added WO3. The sensor that was made of the KOH-added WO3 exhibited favorable impedance, high sensitivity, good linearity, low hysteresis, a short response/recovery time and high long-term stability over the studied humidity range. Complex impedance spectra were used to investigate the humidity-sensing mechanism of the KOH-added WO3.