The determination of the cation exchange capacity (CEC) of clays by exchange with the cationic copper complexes [Cu(en)2]2+ and [Cu(trien)]2+ is revisited. The procedures reported by Bergaya & Vayer (1997) and Meier & Kahr (1999) are modified slightly. The concentration of the copper complexes in the equilibrium solutions is measured by spectral photometry. Correct CEC values of the [Cu(en)2]2+ exchange are only obtained when a buffer ('tris', tris (hydroxymethyl) aminomethane, pH = 8) is added to the equilibrium solution after separation of the clay, because the molar extinction coefficient of this complex depends on the pH of the solution. A standard procedure is recommended as a reference method. In most cases, tris addition is not needed for the determination with [Cu(trien)]2+ cations. Nevertheless, tris addition is recommended in the standard procedure. Determination of CEC for 40 samples (kaolins, 'common clays', bentonites, montmorillonites and beidellite) shows a good agreement between measurements using [Cu(en)2]2+ and [Cu(trien)]2+ cations and with results by the ammonium acetate method.