The use of the molecular probe technique in conjunction with X‐ray photoelectron spectroscopy (XPS) for the assessment of acid‐base properties of polymer surfaces is reviewed. The method is based on the determination of the concentration and chemical shifts of Lewis acids (bases) sorbed in polymers of basic (acidic) character. In the case of chloroform (Lewis acid) sorbed in polymers of Lewis basic character,C12pbinding energy is linearly correlated with &Dgr;HAB, the heat of acid‐base complex formation chloroform‐polymer. This relationship has been used to determine the acid‐base properties of poly(phenylene oxide), a homopolymer, and ammonia plasma‐treated polypropylene. This work shows that XPS can now indeed be used to quantitatively assess the acid‐base properties of modified polymer surfaces and perhaps be extended to map acid‐base properties of polymer surfaces at the micron or submicron scale. ©1996 American Institute of Physics.