This article reports the results of diffusion studies of 4,4'-diphenyl methane diisocyanate (MDI) at 40 degrees C into different kinds of thermoplastic materials, and, thereafter, application of the same concept in polymer joint healing at interfaces. Two categories of samples were used for adhesion studies: ( 1) aged at 40 degrees C for 100 h and ( 2) non-aged samples. The adhesion/failure strength at the interface was measured using a peel test method. The thermoplastic materials used were a copolymer and/or blends of the following polymers: polystyrene-co-maleic anhydride (SMA), polycarbonate ( PC), acrylonitrile-butadiene-styrene (ABS), and silicone-acrylate rubber (SAR). A gravimetric method was used to measure the mass uptake and the optical microscopy was employed for the determination of the diffused layer of MDI into the thermoplastic materials. In two of the thermoplastic blends, glass fiber (GF) was also blended along with the copolymers. MDI was found to have no diffusion in neat/pure SMA copolymer, whereas the diffusion coefficient for PC/ABS was the highest for the samples investigated. The results are discussed in terms of functional groups present in the different interacting materials. In general, the aged samples showed higher adhesive strength than the non-aged samples.