Waterborne polyurethane (WPU) adhesives modified by polystyrene (PS) were prepared through a pre-polymer mixing process from diisocyanates, an anionic polyester, internal emulsifiers, a neutralizer, a chain extender, and PS dispersions. The latter was preformed via the in situ polymerization of styrene in poly(1,4-butanediol adipate) diol. Transmission electron microscopy, Fourier transform infrared spectroscopy, and H-1-NMR techniques were used to characterize the PS dispersions and polyurethane (PU)-PS prepolymer. Experimental results with respect to the performance of the PU-PS adhesives indicate that suitable PS/polyester diol weight ratios improved the mechanical properties, thermal stability, water resistance, and initial adhesive strength of the pristine WPU adhesives. (c) 2008 Wiley Periodicals, Inc.