The fire retarding performance of 28 different inorganic chemical substances was tested by measuring the relative particle fire hazard properties of Pinus halepensis needles treated with these chemicals. The tests were performed using a new method, based on a specifically designed apparatus for monitoring the forest species temperature, under precisely controlled temperature and static air atmosphere conditions. The relative ignition and smoldering combustion properties determined were: the ignition delay time, the combustion rate, the heat content and the mass residue of forest samples. The key elements for the effectiveness of fire retardants were the delay of ignition and the reduction of heat and combustion rate. The chemicals examined were: Cu, Fe, Al2O3, Fe2O3, SiO2-H2O, NaHCO3, KI, KBr, KCl, NaCl, CaCO3, MnSO4-5H(2)O, CuSO(4)(.)5H(2)O, MgCl2(.)6H(2)O, Na(2)B(4)O(7)(.)10H(2)O, Na2HPO4, Na2CO3, Na2SiO3, ZnSO4-7H(2)O, Zn-3(PO4)(2)(.)2H(2)O, NH4Br, NH4Cl, NH4HCO3, (NH4)(2)CO3, NH4H2PO4 (MAP), (NH4)(2)SO4 (AS), (NH4)(2)HPO4 (DAP) and a commercial retardant containing both DAP and AS. Among them the best performance was exhibited by ammoniac phosphates, followed by ammoniac sulfates and silica. (c) 2006 Elsevier B.V. All rights reserved.