The efficiency of removal of the free iron from kaolinite-group minerals by means of acid leaching was followed by chemical, AES, XRD and ESR analyses. Structural Fe responsible for the g = 4 ESR signal represented only a small fraction of Fe impurity in the natural samples. The remaining non-structural Fe was found to exist both as forms easily extractable with acid, as well as deposits that were moderately resistant to the acid treatment. At least part of the latter could be associated with the broad g ~- 2 ESR signal typical of clustered Fe 3+ ions, which in most samples persisted even after prolonged acid leaching. The data demonstrate that procedures commonly used to clean kaolinite of free iron are by no means 100% efficient. In particular, frequently encountered quantitative association of the Fe remaining in the deferrated solids with purely structural Fe, homogeneously distributed in the kaolinite matrix, is probably significantly overestimated and should be treated with great caution.