AbstractThe influence of certain phosphorus substituted (OH, OMe, Me) 3,3'‐diphenylphosphine oxides on flammability and thermal stability of 1,3‐phenylene isophthalamide copolymers was investigated. Introduction of the above phosphorus groups along the backbone of poly(1,3‐phenylene isophthalamide) was performed in a random manner to a degree of ca. 5‐15 mol %. From the limiting oxygen index (LOI) measurements on copolymers, it was found that the diphenylphosphinic acid group has the highest flame retardance efficiency with an increase of LOI 44‐56%. From thermogravimetric analysis (TGA), it was found that the action of phosphorus groups is limited in the solid phase, increasing the char yield to 27‐42%. This was attributed to crosslinking reactions at the stage of amide bond decomposition. © 1992 John Wil