The compositional effect of NaF and CaO/SiO₂ on the hydrogen solubility in the NaF–CaO–SiO₂–FeO_t welding flux system at 1823 K is presented. At a CaO/SiO₂ of 1.3, higher NaF decreased the hydrogen solubility and at a CaO/SiO₂ of 1.5, higher NaF had relatively little effect on the hydrogen solubility in the flux. The hydrogen solubility with CaO/SiO₂ showed a parabolic behavior showing a minimum near unit basicity and increasing with higher basicity. FTIR (Fourier transform infra red) and Raman spectroscopy of as-quenched flux samples showed the addition of NaF and higher CaO/SiO₂ depolymerized the flux structure. At low and intermediate basicities from 0.9 to 1.3, higher NaF decreased the incorporated hydroxyl sites of Si(OH) and FeO(OH), which correlates well to lower hydrogen values.