This thesis describes studies of the relationship between the molecular structure of a range of organophosphorus-based polymer additives and their ability to confer intumescent flame retardant properties on the resulting polymers. The development of intumescent fire retardants is discussed along with the principles of flame retardancy in general. Much of the work centred around the chemistry of a key starting material, pentaerythritol phosphate (PEPA). This compound was found to be less reactive than expected, due to a combination of its neopentyl type structure and the electron withdrawing effect of the phosphoric ester functionality. Various derivatives of PEPA were synthesised, most containing reactive functional groups which facilitated future development of the compound. The derivatives containing no reactive groups were investigated for their intumescent behaviour in their own right. Derivatives of PEPA containing an acidic functionality were utilised in the production of intumescent salt systems using cations with a high nitrogen content in the form of s-triazines. The most promising was the trimethylolmelamine salt of a bis-PEPA derivative of phosphoric acid, which, on testing, proved to be more effective than the current "state of the art" intumescent additive. Metal salts of acidic PEPA-derivatives were also investigated. Derivatives of PEPA containing a carbon-carbon double bond were investigated for their potential to polymerise and thus form more stable additives. Only one such polymer was successfully synthesised, that being poly (PEPA methacrylate). Despite being non-intumescent, due to its high thermal stability this polymer has potential as a flame retardant additive. The monomer was found to copolymerise with methyl methacrylate to form a polymer of high thermal stability. When investigating the relationship between the molecular structure of the compounds developed and their intumescent behaviour, it was observed that only compounds containing an ionisable hydrogen atom exhibited intumescent decomposition on pyrolysis.