The extensibility of poly(diphenylamine) (PDPA) is investigated via Langmuir monolayers and Langmuir-Blodgett (LB) film techniques. PDPA monolayers were prepared by spreading stock solutions of various concentrations on subphases of water, hydrochloric acid (HCI) solution, and methane sulfonic acid (MSA) solution. The effects of various PDPA concentrations and subphases on the behavior of monolayer were analyzed via properties of surface pressure-area per repeat unit (pi -A) isotherms and also observed by using a Brewster angle microscopy. PDPA LB films were transferred onto hydrophilic glass plates and observed by atomic force microscopy. The results show that the extension of polymer chain for PDPA at the air/liquid interface is strongly affected by the PDPA concentration in the spreading solvent. For the various subphases used, PDPA molecules can extend better in an acidic subphase than in the pure water due to the protonation of amine groups at the presence of hydrogen ion. Besides, it is also found that MSA has a superior ability, than HCI, in extending the PDPA molecules and obtaining a stable monolayer. (c) 2005 Elsevier B.V. All rights reserved.