The closed-loop phase diagram of poly N-isopropylacrylamide (PNIPA) in a water-N,N-dimethyl formamide (DMF) system was measured by thermooptical analysis (TOA). The reentrant swelling behavior of N-isopropylacrylamide (NIPA) nano-sized gel particles in the water-DMF system was measured by using photon correlation spectroscopy (PCS) technique. Theoretically, a modified double lattice model (MDL) can be used to describe the closed-loop phase behavior of linear PNIPA in water-DMF systems. For crosslinked NIPA nano-sized gel particles in a water-DMF system, we combined MDL theory for the mixing contribution and Flory-Erman theory for the elastic contribution. Molecular interaction parameters obtained from the PNIPA solution were used to directly predict the swelling-ratio curves for the NIPA gel. Using our model, the calculated results were in good agreement with the experimental data using only one adjustable parameter. (C) 2009 Elsevier Ltd. All rights reserved