Using a micelle-like star polymer as both stabilizer and reductant, a one-pot procedure to ultrastable, narrow-sized and pH-responsive Au nanocomposites in aqueous solution was described. The water-soluble copolymer was composed of a hydrophobic alanine-based hyperbranched poly(ester amide) (HBPEA) core containing thioether moieties, and numerous hydrophilic poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) arms. The process was proposed that the Au ions were firstly coordinated and reduced by the amino groups in PDMAEMA arms, and then transferred to thioether ligands in the HBP core, driven by higher affinity of sulfur atom to Au than amine ligand. Growth of the sequentially reduced Au atoms from these nucleis contributes to formation of narrow-sized and ultrastable AuNPs. The size of the prepared AuNPs shifted in response to PDMAEMA arm length, especially pH, indicating that the star-shaped polymers acted as not only a size-tunable nanoreactor but also a pH-sensitive matrix. The nanocomposites were employed in situ as the catalysts for the reduction of p-nitrophenol, exhibiting excellent catalytic activities under mild conditions due to efficient encapsulation of the substrate to the large void spaces near the AuNP surfaces by the HBP core. The thermo-responsible property of the nanocomposites was favorable of catalyst recycle and other applications. (C) 2012 Elsevier Ltd. All rights reserved.