The assessment of the modes of action of flame retardants (FRs) and their quantification is a challenging issue. This study highlights that the condensed phase efficiency θ can be assessed by combining different flammability tests (pyrolysis-combustion flow calorimeter PCFC, Limiting Oxygen Index LOI, cone). Relations were proposed to calculate LOI and pHRR in cone calorimeter from several parameters including θ. A satisfying agreement between experimental values and calculated ones were found for a set of several dozens of FR-free polymers assuming that θ=1 (i.e. no action in condensed phase).Then, θ was evaluated for EVA/PE blends containing Aluminium TriHydroxyde (ATH) and/or Magnesium DiHydroxyde (MDH) and for ABS containing Ammonium PolyPhosphate (APP) and/or DecaBromoDiPhenylOxide (DBDPO) assuming that LOI and pHRR of flame-retardant compositions follow the same relationships as those of unfilled polymers. The evolution of θ (as well as Χ, i.e. the combustion efficiency) highlights the modes of action of FRs and can contribute to the discussion about possible synergy or antagonism between FRs. Especially, an apparent synergism between APP/DBDPO is observed in cone calorimeter from a performance point of view. Nevertheless, the calculation of χ and θ challenges this conclusion.