We have observed the transient behavior in the AlGaN photoluminescence. Under an excitation of 325 nm He-Cd laser beam, the blue luminescence (BL) bands and yellow luminescence (YL) bands of AlGaN vary with increasing illumination time. We propose that the chemical reactions between BL-related CNON-Hi (CN-Hi) and YL-related CN-Hi (CN) defect states are the cause of such a phenomenon. The BL transition temperature (Tt) is defined as the temperature at which the intensity of BL bands induced by CNON-Hi is equal to that originated from CN-Hi. Only at Tt, BL shows a peak energy variation due to the exposure. The Tt of AlGaN is higher than what is similarly detected in GaN because of the high reactivity of Al to O.