In this study, we investigated the influence of hydrophobized chitosan on the formation and thermodynamic and surface tension properties of insulin–chitosan (I–Ch) polyelectrolyte complexes (PECs). We used an alkylation procedure to insert 12 carbon chains along the chitosan macromolecule with final substitution degrees of 5, 10, and 50%. NMR and IR spectroscopy were used to evaluate the success and extent of the hydrophobization procedure. Isothermal titration calorimetry (ITC) was used to determine the type and extent of the existing intermolecular interactions between the different constituting components of the insulin–hydrophobized chitosan PECs. Through the surface tension and diffusion coefficients at the air–water interface and ITC experiments with different I–Ch proportions, we demonstrated that around 34, 24, 25, and 60–80 insulin molecules saturated 0, 5, 10, and 50% hydrophobized chitosans, respectively. Surface tension experiments at the air–water interface demonstrated that the interaction of insulin molecules on the unmodified chitosan increased the hydrophobicity; this was mainly due to electrostatic interaction. On the contrary, insulin–hydrophobized chitosan interaction lowered the PEC hydrophobicity because of insulin alkyl chain interaction, and therefore, the hydrophilic insulin groups at the PEC surface contributed to a higher surface tension. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014, 131, 39999.