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Glass formation in the system $PbO-PbF$, has been investigated. The structure of these glasses has been studied using X-ray diffraction. Densities, heat capacities, glass-transition and crystallization temperatures and Vicker’s microhardnesses have been measured. D.c. conductivities of these glasses have also been measured as a function of temperature. A structural model has been developed which suggests the existence of $[PbO_2F_4]$-type units over the entire composition range. It is suggested that covalent linkages of the type $-0-Pb-0-$ play a crucial role in determining the composition limits to glass formation. The structural model has been shown to be consistent with other physical properties of the glasses.