It is known that alkali lignin and lignosulphonates possess surface active properties due to their amphiphilic chemical nature. They are often used for controlling the stability and rheological properties of various dispersing systems. Compared with traditional surfactants, knowledge of the behaviour of lignin and their modified products at the air-water interface is more limited. The purpose of this study was to investigate surface films, formed with polyelectrolyte complex nanoparticles (PECNs) from alkali lignin (Lig) and chitosan (Ch) at the air-water interface depending on the molecular mass of Ch, pH values of the reaction mixtures and the time of the polyelectrolyte interaction, using Langmuir technique. A non-stoichiometric polyelectrolyte complex consisting of alkali lignin (Mw = 4160) and chitosan (DD = 75-78%, Mw = 178–400 kDa) was obtained by mixing both polyelectrolytes in aqueous solutions at the defined concentrations and the defined charge ratio (n-/n+ <1)