A series of copolyimides containing different ratios of fluorine and phosphine oxide were synthesized. The copolymers were characterized with Fourier transform infrared, differential scanning calorimetry, and thermogravimetric analysis measurements. The copolymers were thermally stable up to 700 K and exhibited glass-transition temperatures in the range of 495-562 K. The glass-transition temperatures of the copolymers decreased with an increase in the phosphine oxide content. The thermal decomposition behavior of the copolymers was investigated. The copolymers with higher phosphine oxide contents displayed lower onset decomposition temperatures and char yields. A new method involving the multiple-rate isotemperature was used to define the most possible mechanism [G(alpha)] for the reactions. The overall kinetic model function of the thermal decomposition of these copolymers obeyed the Avrami-Erofeev model equation, G(alpha) = [-ln(1 - alpha)](1/m) where alpha is the conversion degree. The apparent kinetic parameters of the degradation processes were also obtained. (c) 2005 Wiley Periodicals, lnc.