The extraction properties of calix[4]arenes, substituted by CH ₂P(O)Alk₂-group on the upper rim were studied. Extraction of Am and Eu by solutions of these substituted calix[4]arenes in m-nitrobenzotrifluoride, dichloroefhane, methylen chloride and p-xylene from nitric acid was performed. It was shown that extraction efficiency of calix[4]arenes from the diluted (C ≤ 0.3 M) nitric acid decreases according to substituent type at phosphorus atom in the following order: butyl ≈ propyl > ethyl > methyl. Such behavior is not typical for corresponding monodentate compounds, so it was preliminary called Anomalous Alkyl Effect (AAE). In order to find out the reasons of this effect the influence of calixarene structure, changing of diluent and the distribution of extractant between water/nitric acid and organic phases were studied. We have stated that CIP-45 (with ethyl substituents at phosphorus atom) transfers into water and nitric acid (C ≤ 0.3 M) from m-nitrobenzotrifluoride solution for more than 80%. CIP-45 also transfers from dichloroethane solution: about 100% transfers into water and less than 10% transfers into 0.01 M HNO₃. This compound fully transfers from p-xylene even into 0.3 M HNO₃ solution. Less than 5% of CIP-67 (with propyl substituents at phosphorus atom) transfer from organic phase (in any of used diluent) into water/nitric acid. IR-spectroscopy investigations of CIP-45 aqueous solutions displayed that the main reason of the extraction efficiency decreasing of these substituted calix[4]arenes from the diluted nitric acid may be the formation of calixarene hydrate. This hydrate is soluble in water, but already in diluted HNO ₃ it transforms into water insoluble solvate.