Abstract A carbonate intercalated Mg/Al layered double hydroxide (LDH-CO3) was subjected to a calcination–rehydration treatment in order to intercalate two different phosphorus containing anions (HPO4− 2 and bis(2-ethylhexyl) phosphate, HDEHP−) into the hydroxide layers. The X-ray diffraction analysis showed that the anions were successfully intercalated presenting different crystallinity degree and crystallite size, being LDH-CO3 the most crystalline, followed by LDH-HDEHP and LDH-HPO4. The effect of the addition of unmodified and modified LDH in different quantities (1, 3 and 5 pphp) to a flexible polyurethane foam matrix containing a castor oil based polyether was studied. Depending on the crystallinity of the LDH, different dispersion degree was achieved in the nanocomposites affecting the properties differently. Density and firmness increased with the addition of LDH while lower resilience values were obtained in foams containing 3 pphp of LDH-CO3 and LDH-HPO4. In addition, thermal behaviour of nanocomposites was analysed by thermogravimetric analysis (TGA) and pyrolysis flow combustion calorimetry (PCFC) and was compared to the reference foam. A decrease in heat release rate was observed in the second stage of degradation, especially for LDH-HDEHP containing foam.