Phenolic resins with improved/altered thermal stability and flame retardancy were prepared by introducing flame retardant elements such as phosphorus and bromine at the monomer stage, followed by condensation with formaldehyde or hexamethylene tetramine (HMTA). Thus, monophenyl phosphoric acid (MPPA), monophenyl phosphoric acid-formaldehyde resin (MPPAF), brominated phenol-formaldehyde resin (BrPF) and 2,4,6-tribromophenyl-phosphoric acid-formaldehyde resin (BrMPPAF) were prepared and characterised. Thermal stability and flammability of these polymers were evaluated by TGA and LOI respectively and compared with those of phenol-formaldehyde resin (PF) and phosphorylated phenol-formaldehyde resin (PPF). The TGA (in air) of MPPAF showed enhanced thermal stability (char yield 66%) over that of PF (char yield 52%) and PPF (char yield 58%) indicating the role of phosphorus in imparting thermo-oxidative resistance to PF. The enhancement is better when phosphorus is introduced at the monomer stage. The incorporation of bromine, however, brought about an unexpected reduction in the thermal stability of PF (char yield : BrPF, 2%; and BrMPPAF, 26%). Both phosphorus and bromine are found to enhance the LOI values of PF (cured) from 35 to 50, 48, and 56, respectively, for MPPAF, BrPF, and BrMPPAF. A comparison of LOI values of these resins with their char yield suggests a condensed phase mechanism for phosphorus and a vapor phase mechanism for bromine operating during thermal decomposition.