The FOO radical was studied at the coupled-cluster (CC) theoretical level, employing both the perturbative calculation of the contribution of triple excitations, CCSD(T), and the exact evaluation, CCSDT. Two solutions were found, one of them with a large spin contamination. It is shown that spin contamination is a problem at the CCSD(T) level, while it is largely immaterial at the complete CCSDT level. The full CCSDT level affords an FO distance of 1.632+/-0.005 A in reasonable agreement to experiment. The enthalpy of formation at the uncontaminated CCSDT level, Delta(f)H(298)(0) (FOO) = 6.5 +/- 1 kcal/mol, is also in very good agreement with the experimental value of 6.24 +/- 0.5 kcal/mol. Contrary to previous studies, CCSDT performs better than CCSD(T) in the calculation of the properties of this radical. (C) 2003 Elsevier B.V. All rights reserved.