4-Arylazo-3-aminophenols (AAPs) were prepared and polycondensed with formaldehyde in the presence of aqueous oxalic acid. The resulting poly(4-arylazo-3-aminophenol-formaldehyde)s (PAAP-Fs) were characterized by yield, melting point, colour, elemental analyses and visible and i.r. spectroscopy. Their M ̄ n values were estimated by non-aqueous conductometric titration and viscometric measurements were carried out in DMF. The thermal properties of PAAP-Fs were examined by thermogravimetric analysis. Their dyeing performance on nylon and polyester was assessed. The dyeing on nylon and polyester had yellow, orange, red and brown shades with moderate to good light and wash fasteness. Polymerizations of AAPs with formaldehyde on dyed nylon and polyester have also been carried out. The dyeing properties of polymeric dyes were compared with the dyes polymerized on nylon and polyester.