Holmium nitrate pentahydrate was reacted with tri-n-butyl phosphate in supercritical carbon dioxide at 308 K. The products of the complexation reaction were measured under supercritical fluid conditions using UV-vis spectroscopy. The solubility of the metal complexes in the supercritical fluid phase was measured. The mole-ratio titration method was used to determine the stoichiometry of the soluble complexes. Conditional extraction coefficients were calculated from spectral data using least-squares regression and hard-equilibria models. Data indicate that the holmium nitrate-tributyl phosphate system forms 1:2 and 1:4 holmium-tributyl phosphate complexes. Conditional extraction coefficients were calculated as log K-ex2 = 5.8 +/- 0.1 and log K-ex4 = 3.1 +/- 0.1 giving a log K-ext value of 8.9 +/- 0. 1. The conditional formation constant for the reaction ML2(scf) + 2L reversible arrow ML4(scf) + 2H(2)O in the supercritical fluid phase was calculated as log beta(204(scf)) = 2.4 +/- 0. 1. (c) 2005 Elsevier B.V. All rights reserved.