This study investigates different types of model phosphorous compounds for flame retardancy purposes to find new FR structures to flame retard rigid poly-isocyanurate (PIR) and/or polyurethane (PUR) foams. Triethylphosphate (TEPate) has been taken as a reference since it is already known to enter into the composition of some FR systems for PIR or PUR foams. Commercially available phosphate, phosphonate, phosphite and phosphine oxide were chosen. All the foam systems were characterized using TGA analysis, rheology tests, and mass loss cone tests, and analyzed using solid state NMR and chemical analysis. DEPonate shows the best efficiency for improving the FR properties of PIR and PUR foam. For the other FRs, the effect on the fire behavior is less relevant especially for PUR foams. This behavior was explained by the fact that even if the FRs slightly decrease the pHRR, the THR or the flaming time, smoke production is usually strongly increased. The systematic study of the condensed phase demonstrates that even if a charring effect is induced by the FR, the effect is not strong enough to improve the FR properties of the material. DEPonate have the least developed char but the best FR properties in PIR and PUR foam, demonstrating that the gas phase effect is more predominant than the condense phase effect.