The fire retarding performance of The retretardancy effect of (NH4)2HPO4 (DAP) and (NH4)2SO4 (AS) in various concentration levels (5%, 10%, 15% w/w) was tested by measuring the relative particle fire hazard properties of various forest species samples treated with these chemicals. The forest species selected: Cupressus sempervirens (Italian cypress), Olea europaea (Olive tree), Pinus brutia (Calabrian pine), Pinus halepensis (Aleppo pine), Pistacia lentiscus (Mastic tree), Quercus coccifera (Holly oak), are very common in the Mediterranean region which is frequently devastated by forest fires. 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 relative self ignition temperature, the combustion duration, and the heat content 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. Both retardants were found to be efficient on different flammability properties during the smoldering combustion of the forest species examined.