Conductive polymer (poly(3,4-ethylenedioxythiophene)- poly(styrenesulfonate) (PEDOT: PSS) is an attractive platform for the design of flexible electronic, optoelectronic, and (bio) sensor devices. Practical application of PEDOT: PSS often requires an incorporation of specific molecules or moieties for tailoring of its physical-chemical properties. In this article, a method for covalent modification of PEDOT: PSS using arenediazonium tosylates was proposed. The procedure includes two steps: chemisorption of diazo-cations on the PEDOT: PSS surface followed by thermal decomposition of the diazonium salt and the covalent bond formation. Structural and surface properties of the samples were evaluated by XPS, SEM-EDX, AFM, goniometry, and a range of electric and optical measurements. The developed modification procedure enables tuning of the PEDOT: PSS surface properties such as conductivity and optical absorption. The possibility to introduce various organic functional groups (from hydrophilic to hydrophobic) and to create new groups for further functionalization makes the developed procedure multipurpose. (C) 2017 Wiley Periodicals, Inc.