Fire retardant treated timber has been used in interior and exterior building structures to satisfy the legal flammability requirements. Dricon and NCX are two commercial phosphorus-based products sold by Arch Timber Protection. However, modifications to their formulation may be required due to their boron and formaldehyde contents, respectively. This research aimed to acquire an understanding of the effect of a number of phosphorus-based fire retardants on the decomposition, flammability and burning behaviour of timber, in order to aid their development. Materials have been investigated on a micro-, bench- and intermediate-scale, and evaluated for physical properties. Thermal decomposition has been studied using thermogravimetric analysis in both air and nitrogen, and simultaneous thermal analysis coupled with Fourier transform infrared spectroscopy. Flammability and burning behaviour has been studied using microscale combustion calorimetry and cone calorimetry. Residue analysis has been carried out using scanning electron microscopy coupled with electron dispersive x-ray analysis. Existing fire retardant timber treatments were applied to timber specimens. All three treatments operated in the condensed phase by reducing the onset temperature of timber pyrolysis, and promoting the formation of a stable char and water. Scanning electron microscopy revealed a barrier formation on the surface of Dricon treated char. All treatments increased the CO:CO2 ratio during burning in the cone calorimeter. This is attributed to reduced volatile gas production, enabling simultaneous glowing oxidation of the char surface. Depth of penetration studies showed that high concentrations of phosphorus on the surface of timber is sufficient to significantly reduce the fire growth rate index, but deeper penetration and increased overall phosphorus concentration is more effective at reducing the total heat released by the substrate. New phosphorus-based formulations were applied to timber and their effect on its flammability was assessed. All treatments reduced the total heat released, but not all were effective enough to be used commercially. Ammonium hypophosphite increased the residue yield of timber and was shown to operate via both condensed and gas phase fire retardancy mechanisms. A low melting glass formed a gel-like barrier on the surface of the char, but its melting point was too high to optimise the reduction in peak heat release rate. Both ammonium polyphosphate and guanidine/ammonium phosphate formulations promoted carbonisation of the timber structure. A coherent barrier layer was formed by the organoclay; however, the improvement was not effective enough to warrant further investigation. Combinations of nanometric oxides and phosphoric acid were effective. The oxides catalysed the phosphoric acid mechanism to promote the pyrolysis of timber and re-radiating mechanisms were proposed for the char. Further work is suggested to improve the effect. Layer by layer assembly does not provide a sufficient loading to effectively reduce the flammability of timber. Lastly, attempts were made to address the problems of scale-up between micro scale thermal decomposition, bench-scale burning and intermediate-scale regulatory fire tests. Three models for the prediction of Euroclass results have been compared and applied to the materials investigated within this thesis. A method is proposed for the prediction of the Euroclass of fire retarded timber products. Correlation coefficients between micro-, bench- and intermediate-scale flammability tests have been calculated and the results are discussed.