Preparation and characterization of nanoparticles from L-phenylalanine substituted dextrans (Dex-Phes) and carboxyl-terminated poly(epsilon-caprolactone) (PCL-COOH) blends in water/dimethyl sulfoxide (DMSO) (10/1, v/v) solution are reported. Dex-Phe with degree of substitution (DS) 0.67 has good solubility in both water and DMSO, which can form the nanoparticles with PCL-COOH in water/DMSO solution. The size and size distribution of the nanoparticles were characterized by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The effect of weight ratio of PCL-COOH to Dex-Phes on particle size was investigated. A possible mechanism for the formation of nanoparticles was also proposed. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 830-836, 2011