The dynamics of domain wall motion in GdyY3−yGaxFe5−xO12garnet films has been studied using optical response to a fast‐rise pulse in bias field. Damped oscillations of the domain walls have been observed and analyzed to yield domain wall mass per unit area, mobility, and limiting velocities. Masses have been found which are in excess of the accepted theoretical values for thick samples by factors between 1.5 and 3.6. Limiting velocities have been determined in general agreement with values predicted by Slonczewski for motion involving generation and propagation of horizontal Bloch lines. The application of a magnetic field in the plane of the film and in the plane of the domain walls under observation caused a decrease in domain wall mass density and increases in mobility and limiting velocity. The mass decrease is substantially greater than that predicted by theory.