Compositions based on self-extinguishing expandable vinyl aromatic polymers in granules comprising: a. a polymeric matrix and, homogeneously englobed in the polymeric matrix, b. 3-10% by weight, calculated with respect to the polymer (a), of an expanding system; c. 0.005-5% by weight, calculated with respect to the polymer (a), of a brominated flame-retardant agent; d. 0.001-2% by weight, calculated with respect to the polymer (a), of a synergic additive; e. 0.005-5% by weight, calculated with respect to the polymer (a), of at least one stabilizing additive selected from: i. pyrophosphates of alkaline or alkaline earth metals, an ammonium group or a unit deriving from melamine; ii. melamine polyphosphate; iii. partially or completely salified polycarboxylic acids; iv. citrates of alkaline or alkaline earth metals; v. polyfunctional alcohols with 2 or more alcohol functions; vi. esters of poly-functional alcohols.