Thermal analysis is widely used for the investigation of the thermal decomposition of polymeric/lignocellulosic materials. Differential thermogravimetry (DTG) curves have been used for the assessment of fire retardants employing a mathematical technique, based on the observation that the DTG profile peaks become less pronounced and are shifted to lower temperatures when a fire retardant is added. The efficiency of a fire retardant is proposed to be assessed from a formula comprising of the following: (i) the depth of the main peak; (ii) the main decomposition peak temperature; (iii) the temperature range of the DTG curve; and (iv) the area above the curve. Initially, four well-known fire retardants have been assessed for their efficiency on a lignocellulosic matrix (Olea europaea). All the results deriving from the mathematical technique have been compared with the mass residue criterion and a limiting oxygen index test (Relative Limiting Oxygen Index). Both are commonly used as reliable tools for the assessment of a fire retardant. Secondly, the impact of fire retardant concentration on efficiency was investigated. In addition, using mathematical routines, an optimum concentration zone was proposed; further, an optimum concentration value (%) has been estimated. Copyright © 2014 John Wiley & Sons, Ltd.