The effect of the polymer molecular weight on the interaction between pentaethylene glycol n-octyl ether (C8E5) and poly(acrylic acid) (PAA) has been investigated by a combined experimental strategy including tensiometry, potentiometry, calorimetry, fluorescence quenching and intradiffusion (pulsed gradient spin echo-NMR) measurements. PAA samples with an average molecular weight varying in a wide range ((M) over bar (w) = 2000, 100,000, 250,000, and 450,000) have been considered. The measurements have been performed at constant polymer concentration (0.1% w/w) with varying surfactant molality. In all the considered systems, at low surfactant concentration, adsorption of surfactant monomers onto the polymer chain has been detected. At a C8E5 molality (T-1) independent of the PAA (M) over bar (w), surfactant molecules start to aggregate, forming clusters to which the polymer co-participates. Above this concentration, the behavior of the system depends on (M) over bar (w). In fact, if polymer samples with high molecular weight ((M) over bar (w) >= 100,000) are employed, all the added surfactant aggregates onto the polymer leading to the polymer saturation and, subsequently, to free micelles formation. Both saturation and free micellization occur at surfactant concentrations which are independent of the polymer molecular weight. C8E5 aqueous mixtures containing PAA with low molecular weight ((M) over bar (w) = 2000) behaves differently, in that, above T-1, only a fraction (similar to 20%) of the added surfactant molecules interact with the polymer, forming aggregates to which more than one PAA chain participate. In this case, C8E5 free micellization occurs before polymer saturation. The experimental evidences have been interpreted in terms of the subtle balance between the various molecular interactions driving the surfactant-polymer aggregation. (C) 2007 Elsevier Inc. All rights reserved.