Precise measurement of the content and the layer charge of illite and smectite is an important aspect of the mineralogical calibration of geophysical well logs and of the evaluation of mechanical and chemical properties of sedimentary rocks. A technique for obtaining such measurements was developed during mineralogical studies of the Miocene clastic rocks from the Carpathian Foredeep. X-ray diffraction (XRD), chemical, cation exchange capacity (CEC), H2O sorption, and ethylene glycol monoethyl ether (EGME) sorption data were obtained for 65 samples of sandstones, shales and carbonates. The illite + smectite sum, involving all detected 2:1 minerals (smectite, illite-smectite, illite, glauconite and muscovite) was measured by XRD. The content of the illitic component was evaluated separately using % K2O and accounting for % K2O in K-feldspar. The content of the smectitic component was estimated from EGME retention using the reference data for a smectite standard, and from CEC assuming the smectitic layer charge of 0.41/O10(OH)2 (Środoń et al., in press). All these measurements produced very consistent results.