Cobalt (Co) nanoparticles (with different loadings, 1 and 2 wt %, of Co) were synthesized in situ in a poly(vinyl alcohol) (PVA) matrix with and without meta-nitroaniline (m-NA) as a dopant (2.5 wt %). The obtained nanocomposite films were characterized with various physicochemical techniques, including ultraviolet-visible spectrophotometry, X-ray diffraction analysis, scanning electron microscopy, and Fourier transform infrared analysis. To study the effect of the humidity, the nanocomposite solutions were coated on planar glass substrates. The beam of an He-Ne laser was incident normal to the film surface and was subjected to variable relative humidities (RHs; 4-93%); the transmitted intensity was measured on a photovoltaic diode. Variations in the intensity of light caused by the changes in RH within the range 3-93% were recorded. We optimized the response by varying the film thickness by coating the solution layer by layer. We generated the RH (4-100%) by passing wet water vapors. The neat PVA film of similar thickness gave humidity sensing in the range 78-93% RH. The sensors with m-NA-doped Co/PVA gave better sensitivity (6.4 mV/% RH) than the undoped samples (1.78-2.45 mV/% RH), exhibiting a fast response of 3 s (2-93% RH) and a recovery of 10 s (93 to 2% RH). These samples also showed reversible behavior and long-term stability (for nearly a year) with a good sensitivity and a large dynamic range (2-95% RH). An attempt was made to explain the results on the basis of a bulk mechanism. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 3565-3574, 2012