Poly(methyl methacrylate)/9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (PMMA/DOPO) compositions have been prepared by extruding and injection molding. The mechanical (i.e., tensile and flexural strengths, modulus, hardness), rheological (volume flow rate index) as well as thermal properties (Tg, Vicat softening temperature, oxygen index) of samples containing 1-10 wt parts of DOPO (per 100 wt parts of PMMA) have been analyzed. Additionally, FTIR analysis of modified PMMA has been made. The chemical reaction of both components has been observed during their processing and PMMA/DOPO compositions exhibited generally higher tensile strength, rigidity, hardness, volume flow rate index, and oxygen index. On the other hand, the addition of the mentioned phosphorous-based component into a PMMA matrix reduced its impact strength, softening, and a glass transition temperature. Moreover, taking into consideration the thermal stability (i.e., temperature for 10 wt% mass loss in the air atmosphere) the mentioned parameter was reduced by a DOPO application as well. POLYM. ENG. SCI., 54:1030-1037, 2014. (c) 2013 Society of Plastics Engineers