The imidization reaction of PMR-15 is monitored in-situ using optical grade sapphire fibers utilizing evanescent wave spectroscopy. To follow the curing reaction, the sapphire fibers are coated with uncured PMR-15 and heated to temperatures up to 200 degree(s)C for varying lengths of time. IR spectra are collected during the cure cycle. The bands that can be used to monitor the reaction are the primary amine bands at 3363 cm-1 and 3462 cm-1, and the secondary amide band at 3478 cm-1. Also used are the ester acid OH band at 4440 cm-1 and the primary amine band at 5050 cm-1. These bands are ratioed with the constant CH band at 4050 cm-1 in order to eliminate uncontrolled experiment-to-experiment variations. The IR spectra show the imidization reaction occurs, very slowly, at temperatures as low as 60 degree(s)C. It also shows that even after 2 hours at 200 degree(s)C, the recommended cure cycle, the reaction is not complete. The focus of this research, to date, is to improve quality control of the cure process to reduce part to part variations in mechanical properties. To this end, an attempt will be made to correlate the absorbance ratio of these bands to mechanical properties of the cured part.