The effects of fluorine and nitrogen substitution for oxygen in a calcium aluminosilicate glass have been examined. Twelve glasses in the Ca–Si–Al–O–N–F system, with constant cation ratio, have been prepared and characterized with respect to density, molar volume (MV), fractional glass compactness (C), Young's modulus (E), microhardness (μHv), and glass transition temperature (Tg). Linear regression analysis of the property data with respect to nitrogen and fluorine substitution for oxygen strongly indicates that each affects property values independent of each other. Nitrogen substitution decreases MV and increases C, E, μHv, and Tg values due to the greater network connectivity associated with tri-coordinated nitrogen. Fluorine substitution, which reduces network connectivity, decreases Tg values as might be expected. However, MV, C, E, and μHv are little affected by increases in fluorine content. The vastly different effects of nitrogen and fluorine on E and μHv are due to the manner in which their substitution for oxygen affects glass free volume and thus fractional glass compactness (i.e. atomic packing density). Differences in the effects of N and F substitution on Tg reflect their different effects on glass crosslink density. A method for calculating crosslink density is presented here, and the need for this method to take into account the ratio of network forming to network modifying aluminum is discussed.