An apparatus is described for measuring thermal and magnetization dynamics during thermomagnetic writing of information onto magneto-optic films. By probing various regions of the reversed spot, the spatial dynamics of bit formation can be reconstructed. The spatial resolution of the measurement is better than 1 μ and the intrinsic temporal resolution is 10 nsec. Representative results obtained with a ferrimagnetic and a ferromagnetic sample are included. © 1974 The American Institute of Physics.