Optical waveguide fluoride glasses face a number of difficult problems despite their potential for ultra-low loss. One of the most serious is scattering loss from defects such as inclusions, phase separation particles and crystallites. By means of a Leitz optical microscope, X-ray diffraction, transmission electron microscope (TEM) and differential thermal analysis (DTA), we have investigated the phase separation and crystallization phenomena in ZrF4-BaF2-LaFs-AlF3 fluoride glasses. Phase separated particles, which formed via the nucleation and growth mechanism, were observed in the glass. The crystallites in the glass are confirmed to be beta-BaF2·ZrF4 and α-BaF2·2ZrF4. The activation energies of crystallization of these phases were determined by DTA.