Indoly-benzylfulgimide belongs to the photochromic fulgide family and follows photochemical first order kinetics. Its bleaching kinetics is investigated at 633nm and 640nm by spectroscopy, by the time dependence of transmission and of diffraction from holographically induced gratings. The non-exponential decay law resulting for diffraction experiments with a Gaussian beam profile is calculated and verified experimentally. For a quasi-homogeneous beam profile the time constant determined from diffraction decay is half the one for absorbance decay. The photochemical reaction rate of indoly- benzylfulgimide in PMMA is (3.9±0.3) cm2/J at 650 nm.