Molecular dynamics simulations on the 1:1 M3+ lanthanide (La3+, Eu3+ and Yb3+) “inclusion” complex of a t-butyl-calix[4]arene L substituted at the narrow rim by four CH2–P(O)Ph2 arms demonstrate the role of hydration and counterions on the cation binding mode and shielding. In dry chloroform and in the absence of counterions, the cation is “endo”, fully encapsulated within the pseudo-cavity delineated by the four phosphoryl arms and the four phenolic oxygens. This “endo” bidentate binding mode is supported by full ab initio quantum mechanical optimization of the calixarene M3+ complexes. In biphasic solution, the complexes are shown to be surface active and to adsorb at an “oil”/water interface with the cationic site pointing towards water and the hydrophobic t-butyl groups in “oil”. The cation is not encapsulated, but adopts an “exo ” position, coordinated to the four P[double bond, length half m-dash]O oxygens of L, to water molecules, and to counterions. This complex is too hydrophilic to be extracted from the interface to an organic phase. The unexpected binding mode has important implications concerning the mechanism of liquid–liquid ion extraction and the microscopic state of the extracted complex in the organic phase.