Thermodynamic quantities were determined for the extraction of ion pairs of tris(1,10-phenanthroline) iron(II) chelate cations with halide, thiocyanate and perchlorate anions from aqueous phase into nitrobenzene. Ion pairs with larger anions have more negative enthalpy changes with higher extractability. For bromide, the enthalpy change was found to be almost zero and a positive change was observed for chloride. Entropy changes are also discussed. © 1969, Taylor & Francis Group, LLC. All rights reserved.