The interfacial reaction of a random copolymer of styrene and maleic anhydride (SMA) with bis(amine)-terminated poly(tetrahydrofuran) (PTHF) is studied by FTIR spectroscopy and microscopy as a function of temperature and for two different PTHF molecular weights. The reaction rate constants in the range from 170 to 230 degrees C, obtained from the initial slope of conversion-time plots, give values between 3.6 x 10(-5) and 2.5 x 10(-4) s(-1). It can be shown that the diffusion of the PTHF into the SMA phase occurs prior to the imide formation. Thus the imide is formed in the SMA phase and can be estimated quantitatively by the appearance of the imide peak at 1702 cm(-1) and the decrease of the out-of-phase carbonyl stretching vibration of the maleic anhydride at 1780 cm(-1). The concentration profiles near the interface of the used polymers and the formed imide are studied by FTIR microscopy.