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The stability of polymer-to-polymer joints was assessed using a thermodynamic method based on surface tension components, similar to that developed by Kinloch et al.,1 showing that joint stability is improved in air by plasma treatments, while it is depressed in water. Shear strength tests showed qualitative agreements with these suggestions.Surface analysis by contact angle measurements, SSIMS and XPS allowed the clarification of contributions which originated from phenomena unaccounted for by thermodynamic analysis. Plasma treatment was shown to form a weak boundary layer on polypropylene surfaces, possibly due to depolymerization. In the case of polycarbonate a water sensitive, possibly water soluble, layer is formed, therefore inducing a remarkable decrease in shear strength upon immersion in water.