This paper presents the development and experimental characterization of a prototype ultrafine particle concentrator. In this system, ultrafine particles pass over a pool of warm water where they become saturated with vapor, and subsequently drawn through a condenser, kept at a lower temperature, that allows the ultrafine particles to grow to super-micrometer size by vapor condensation on their surface. In order to increase particle concentration, the grown particles are drawn through a virtual impactor with an approximate 50% cutpoint at 1.5 mu m. The concentrated particles from the minor flow of the virtual impactor finally pass through a diffusion dryer that removes the excess water on the ultrafine particles and returns them back to their original size and relative humidity. In its optimum configuration, the ultrafine concentrator operates at a sampling flow rare of 106.5 or 1101min(-1) and concentrates the ultrafine particles to 3.5 or 71min(-1) by an enrichment factor of approximately 15 and 25.5, respectively. Our experimental results identified saturation of the ultrafine aerosols at 35 degrees C and cooling to 25 degrees C as the optimum temperatures for operation of the ultrafine particle concentrator. Lower temperatures either do not concentrate, or concentrate less efficiently the ultrafine particles. Increasing the saturation temperature to 40 degrees C and cooling to 31 degrees C does not improve the concentration enrichment achieved by the optimum configuration. Our results also indicated that the concentration enrichment does not depend on the chemical composition of the ultrafine aerosol. Hygroscopic ammonium sulfate, volatile ammonium nitrate, hydrophobic polystyrene latex and actual "real-life" indoor air ultrafine particles were all concentrated by practically the same factor. More importantly, the experimental results show that particle concentration occurs without any coagulation, which would have distorted the size distribution of the original ultrafine aerosols.