Novel impedance-type humidity sensors were fabricated by coating 2,5-dimethyl-1-phenyl-1H-pyrrole (1), 1-(4-aminophenyl)-2,5-dimethyl-1H-pyrrole (2), 1-(4-nitrophenyl)-2,5-dimethyl-1H-pyrrole (3) and gold nanoparticles/compound 3 (AuNPs/compound 3) composite films on alumina and plastic substrates. Compound 1, compound 2, compound 3 and AuNPs/compound 3 composite films were analyzed using atomic force microscopy (AFM) and UV–vis spectroscopy. The effects of the substituents on the benzene ring of compound 1 on the electrical and humidity-sensing properties of the compound 1 film were studied. The humidity sensor that was fabricated with the compound 3 exhibited high sensitivity but a higher resistance at low humidity than that which was made of the compound 1. The effects of the AuNPs on the electrical and humidity sensing properties of the AuNPs/compound 3 composite film on a PET substrate were investigated. The sensor that was made of the AuNPs/compound 3 composite film exhibited high sensitivity, good linearity, small hysteresis, high flexibility, a short response/recovery time, a weak dependence on temperature and high long-term stability. The linearity of the humidity sensor depended on the applied frequency. The sensing mechanism of the AuNPs/compound 3 composite film was elucidated with reference to impedance plots.