The effects of fluorine and nitrogen substitution for oxygen in aluminosilicate glasses, effectively oxyfluoronitride (OFN) glasses, modified by calcium, calcium-yttrium or calcium-magnesium on thermal and physical/mechanical properties have been compared. Thus, 42 glasses in the Ca-(Mg)-(Y)-Si-Al-O-(N)-(F) system have been prepared and characterized with respect to density (rho), molar volume (MV), compactness (C), free volume (FV), glass transition temperatures measured by DTA (T-g,T-DTA) and dilatometry (T-g,T-dil), dilatometric softening point (T-DS), microhardness (mu Hv) and Young's modulus (E). Gradients of property variation with nitrogen or fluorine substitutions for oxygen are similar for all three different oxyfluoronitride glass systems and are comparable with those reported for other OFN glasses, again indicating independent and additive effects of nitrogen and fluorine. In attempting to further understand how fluorine affects the cross-link density (CLD) in OFN glasses, it becomes apparent that it is necessary to allow for a greater contribution by aluminum in a modifier role as fluorine content is increased. This modified calculation of CLD values results in good linear fits between T-g and CLD values. This analysis clearly demonstrates and endorses the concepts that thermal properties are related to CLD while physical/mechanical properties are dependent on glass compactness.