This development describes a design of a high humidity tandem differential mobility analyzer (HHTDMA) to determine aerosol hygroscopic growth factors at relative humidities (RH) between 90% and 98% under highly temperature-stabilized conditions. Two temperature baths are used to thermally stabilize the humidity conditioning section and the second DMA and to maintain a slightly lowered temperature in the second DMA relative to the particle humidity conditioning section. The reduced temperature of the second DMA results in constant and higher RHs between 90% and 98%. We describe the basic system design and results from temperature and RH stability tests. In the HHTDMA-system, the absolute temperatures can be maintained within +/- 0.1 K and with a long-term stability of 0.02 K. The RH in the second DMA reaches an absolute accuracy of +/- 1.2% for 98% and a long-term stability of +/- 0.1-0.4% of set point values. To demonstrate the capability of the HHTDMA, hygroscopic growth factors of ammonium sulfate aerosol between 84% and 98% RH were determined. The experimental results agree with theoretical values within the uncertainties of the system, with values based on measurements using a single-particle levitation technique, and growth factors of other TDMA experiments in the range up to 94% RH. (C) 2005 Elsevier Ltd. All rights reserved.