We report here a new pathway to prepare chitosan (Ch)-capped silver nanoparticles (NPs) in aqueous solutions without addition of any stabilizer and reductant. First, 1 g L-1 Ch was added in a 0.05 N AgNO3 aqueous solution with pH 6.6. Then the solution was heated from room temperature to boiling at a heating rate of 6 degrees C min(-1) to prepare Ch/Ag nanocomposites (NCs). The particle size of prepared Ag NPs with main (1 1 1) face on Ch is ca. 10 nm. Encouragingly, the prepared Ch/Ag NCs are more active catalysts than Ch is for the decomposition of 2 ppm formaldehyde in a dark and sealed chamber. After testing for 24h, the formaldehyde molecules were decomposed by ca. 96% and 65% via employing Ch/Ag NCs and Ch, respectively. (c) 2010 Elsevier B.V. All rights reserved.