Abstract The relationship between clay mineral transformations and copper sorption in a forest soil (Luvisol), as well as the effect of adsorbed Cu ions on clay mineral properties, was studied by analytical transmission electron microscopy (TEM-AEM) and X-ray diffraction (XRD). Vermiculitic soil clay with chlorite/vermiculite mixed minerals and pedogenic Al-vermiculite with partial hydroxy-interlayering adsorbed large amounts of copper, whereas uptake by illite was found to be insignificant. Copper uptake increased with decreasing Mg within the chlorite/vermiculite series. Vermiculitization of chlorite increased copper sorption, but fixed potassium and illitization, as well as initial Al-hydroxy-interlayering of pedogenic Al-vermiculite, slightly reduced copper uptake. Iron coatings may also be an important factor in the increase of copper sorption in clay minerals. XRD revealed that the major part of Cu sorption takes place in the interlayer space of vermiculite components and can cause the loss of vermiculite intracrystalline expansion. AEM analyses provide a useful tool to study the effect of clay mineral transformation on metal sorption in soils.