The cyclic phosphorous group, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide-10-yl (DOPO), attached on a polyamide PA-DOPO could be hydrated to give its open-chained form with phosphinic acid groups (PA-DOPO-HD). The open-chained phosphinic acid groups could be reformed to its cyclic DOPO form through thermal dehydration. The occurrence of the hydration and thermal-dehydration reactions and the chemical structure of the polyamides were demonstrated with FTIR, 31P NMR, and thermal analysis. PA-DOPO-HD exhibited similar inherent viscosity and better organosolubility than the original polyamide PA-DOPO. ; JNL0002920