[en] New types of acidic organophosphonates, HR with different hydrophobic groups, were synthesized in order to investigate their extraction behavior for heavy rare earth elements (Y³⁺, Ho³⁺, and Er³⁺) from aqueous acidic chloride media. Their physicochemical properties, such as aggregation in an organic diluent and the acid dissociation constants (K_a) in the aqueous phase, were also examined. These rare earth elements were supposed to be extracted according to the stoichiometric relation, M³⁺ + 3(HR)₂ ↔ MR₃ · 3HR + 3H⁺. This idea and the extraction equilibrium constants (K_ex) for each metal ion were evaluated. Both the extraction equilibrium constants and the separation factors (β) between these metal ions were found to be greater than those of commercial extractants. The correlation between the extractability of heavy rare earth elements, the selectivity among these elements and the chemical structures of these extractants are discussed from a qualitative perspective. (author)