Symmetric polydisperse (M-w = 23 x 10(4), M-w/M-n = 2.84) and monodisperse (M-w = 21 X 10(4), M-w/M-n < 1.05) polystyrene (PS), and asymmetric polydisperse PS/ poly(2,6-dimethyl 1,4-phenylene oxide) (PPO) interfaces have been bonded in the vicinity of the glass transition temperature (T-g) of PS. In a lap-shear joint geometry, strength develops in all cases with time to the fourth power, which indicates that it is diffusion controlled. Strength developing at short times at the polydisperse PS/PS interface, at 90 degrees C, is higher than that at the monodisperse interface, at 92 degrees C (at T-g - 13 degrees C in both cases), presumably due to the contribution of the low molecular weight species. The decrease of strength at the PS/PPO interface when the bonding temperature decreases from 113 to 70 degrees C, i.e., from T-g + 10 degrees C to T-g - 33 degrees C of the bulk PS, indicates a high molecular mobility at the surface as compared to that in the bulk, and can be expressed by a classical diffusion equation, which is valid above T-g (of the surface layer).