The thermal degradation of wool treated with the flame retardant synergistic system, zirconium acetate, citric acid and formic acid, was studied by thermal analysis, mass loss, limited oxygen index (LOI) and vertical flame test. The surfaces of the wool samples were observed by SEM. The treated wool samples show an increase in the temperature of decomposition, residual mass and LOI in comparison with the untreated wool. The central composite design (CCD) was used for the experimental plan with four variables on the results of flame retardancy. Statistical analysis confirms the optimum conditions obtained by the experimental results. (C) 2011 Elsevier B.V. All rights reserved.