In this work, the effect of processing conditions on the production of an intumescent flame-retardant system is studied in polypropylene-based compounds. Two distinct procedures were used: ultrasonic assisted single screw (with a static mixer die) and twin screw extrusion. The flame-retardant, thermal, mechanical, morphological, and rheological properties were measured. It was found that the flame-retardant intumescent content can be diminished from 30 phr (as usually used) to 21 phr using the application of ultrasonic waves during extrusion and with the addition of chemically modified clay to obtain a V0 classification according to UL94-V standards. In addition, the processed materials presented improvements in the mechanical properties such as impact resistance (Izod Notched), strain at break and tenacity upon ultrasound application. The online application of ultrasound through a die that produces extensional flows improved greatly the dispersion and distribution of the particles of the intumescent system and the chemically modified clay in the polymer matrix. POLYM. ENG. SCI., 2013. (c) 2013 Society of Plastics Engineers