Polyethylene was flame retarded with an intumescent flame retardant at 27 wt.% and expandable graphite at 10 wt.% either on its own or in combination with 10 wt.% or 20 wt.% of the intumescent. Two grades of each flame retardant type were used. They differed primarily with respect to the onset temperature for exfoliation (expandable graphites) or decomposition (intumescents). The fire performance of pressed sheets with a nominal thickness of 3.2 mm was tested in a cone calorimeter at a radiant flux of 35 kW m-2. The best char yield was obtained with compositions containing the high-decomposition-onset-temperature intumescent 3,5-diaminobenzoic acid phosphate. Despite this, the best fire performance overall was realized using the low-decomposition-onset-temperature ethylenediamine phosphate, together with the low-exfoliation-onset-temperature expandable graphite. Formulations based on this intumescent compound ignited later, reached their peak heat release rate earlier, and also burned longer irrespective of the expandable graphite grade used. This can be attributed to the formation, at the burning surface, of a more cohesive char with better thermal and mass transfer barrier properties. This explains why compositions based on ethylenediamine phosphate as intumescent outperformed those containing 3,5-diaminobenzoic acid phosphate in terms of the peak heat release rate and other important fire indices