For the first time, polyester and polyester cotton fabrics have been treated with an aqueous suspension of caseins to increase their thermal stability and flame retardancy. The effectiveness of the fabric treatment as well as the morphology of the deposited coatings have been assessed by infrared spectroscopy and scanning electron microscopy, respectively. The thermal stability of the treated fabrics in nitrogen and air, as well as their resistance to a flame application and to an irradiative heat flux of 35 kW/m(2), has proven to be strongly affected by caseins. Indeed, in the case of polyester, a remarkable decrease of the burning rate (-70%) and a significant increase of its limiting oxygen index (from 21 to 26%) has been reached. In the case of polyester-cotton blends, caseins turned out to slow down the fabric burning rate (about -40%) and its resistance to an irradiative flux as assessed by cone calorimetry.