A novel flame retardant containing phosphorus and sulfur, bis(2-tienyl)phenylphosphine (BTPP) was synthesized and characterized with Fourier transform infrared spectroscopy, 1H, 13C, and 31P nuclear magnetic resonance. BTPP was used to impart flame retardancy to polycarbonate (PC). Combustion behaviors and thermal degradation properties of PC/BTPP system were assayed by limiting oxygen index (LOI), vertical burning test (UL-94), cone calorimeter test, and thermogravimetric analysis. PC/3 wt% BTPP passed UL-94 V-0 rating with 3.0 mm samples and the LOI value was 36.5%. PC/6 wt% BTPP passed UL-94 V-0 rating with 1.6 mm samples and the LOI value was 38.5%. Scanning electron microscopy revealed that char properties had direct effects on the flame retardancy. Mechanical properties and water resistance of PC/BTPP system were also examined. After water resistance test, PC/3 wt% BTPP with 3.0 mm samples and PC/6 wt% BTPP with 1.6 mm samples kept V-0 rating and mass loss were only 0.2%. POLYM. ENG. SCI., 2012. (C) 2012 Society of Plastics Engineers