Liquid-liquid extraction of Eu3+ from aqueous solution with 4-oxaheptanediamides (OHAs) as extractant into room temperature ionic liquids (RTILs) of 1-alkyl-3-methylimidazolium hexafluorophosphate (C(n)mim(+)PF(6)(-), n = 4, 6 and 8) was investigated. The strong affinity of OHAs to Eu3+ was observed in the present C(n)mim(+)PF(6)(-) system. The extraction was assumed to proceed by cation-exchange mechanism and formed a 4:1 complex of the OHA extractants and Eu3+ in C(4)mim(+)PF(6)(-) system. The preferable composition of extracted species was presumed to be Eu(OHA)(4)(H2O)(4)(PF6)(3) by ESI-MS.