The possibility of characterizing dispersion forces and acid-base interactions by means of physico-chemical measurements is demonstrated by the examples of contact angle and zeta potential measurements, with special attention being given to the latter. This method has been applied, to characterize the effect of plasma and flame treatment on the adhesion behaviour of injection moulded poly(propylene) specimens. The results with respect to acidic or basic functional surface sites, as obtained by zeta potential measurements, correlate with the elemental surface compositions determined by XPS. There is no general interrelation between acidic and basic parameters determined by contact angle measurements and the results of zeta potential and XPS measurements.