Phosphorus containing polyimides were prepared via phosphorylation of organosoluble polyimides. This was achieved by phenoxaphosphine oxide ring formation reaction or esterification with diethylchlorophosphate. The phosphorylation was confirmed by infrared, P-31 nuclear magnetic resonance, and elemental analysis for phosphorus. Polyimides containing phosphorus of 8.3 and 5.4% by weight were found. Thermal characteristics and decomposition behavior of the resulting polyimides were investigated by differential scanning calorimetry and thermogravimetric analysis. Introduction of phosphorus into polyimides slightly reduced their initial weight loss temperatures and led to high char yields at temperatures higher than 800 degrees C. Limiting oxygen index values higher than 48 were found for the phosphorylated polyimides. Such properties make these polymers useful in flame retardant applications.