Hyperbranched polycarboxylates (HBPC) with different alkali metal cations (Li(+), Na(+), and K(+)) were prepared and characterized by (1)H-NMR and thermal gravimetric analysis. Thin film humidity sensors based on HBPC and its composite with ZnO nanorods were fabricated. The morphologies of films of HBPC and the nanocomposite were investigated by atomic force microscopy, which revealed uniform distribution of ZnO nanorods in HBPC. The humidity-sensitive characteristics of HBPC and the nanocomposite were investigated at room temperature. It was found that the type of cations significantly affected the humidity-sensing behaviors of HBPC. In addition, the nanocomposite exhibited better humidity-sensitive properties than HBPC alone. Its impedance decreased for about three orders of magnitude over the range 19-97% RH, showing high sensitivity. Moreover, the nanocomposite exhibited fast response (similar to 9 and 10 s for response and recovery time between 97% RH and 33% RH, respectively) and small hysteresis (similar to 1.4% RH). The improved humidity-sensing behaviors of the nanocomposite over HPBC alone is explained by taking into account the hyperbranched structure of the polymer and the special interactions of the polymer and ZnO with water molecules. (c) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 1994-2000, 2011