The effects of the composition and temperature on the hydrogen solubility and melt structure in the TiOx-MnO-SiO2 welding flux system were investigated. The redox reaction between Ti3+ and Ti4+ was distinguished within TiOx based on the substitutional effects of TiOx in SiO2. A higher TiOx content compared with SiO2 decreased the hydrogen solubility because the dominant hydrogen dissolution mechanism changed from an incorporated hydroxyl to a mixed region that included both the incorporated and free hydroxyl mechanisms. The Ti3+/Ti4+ trend and the decrease in the absolute amount of SiO2 appear to cause this mechanistic change. Based on the effects of the basic and acidic components on the hydrogen solubility in the melt, the effect of the extended basicity ((Ti2O3+MnO)/(TiO2+SiO2)) was also analyzed and exhibited a parabolic behavior. Three regions were observed from low to high extended basicity, which correspond to the incorporated to free hydroxyl modification with a mixed region. FTIR analysis of the as-quenched melts provided insight into the changes in the Si-O tetrahedral stretching bands and Si-OH bending, which were more pronounced as the SiO2 content increased. The FTIR and XPS results obtained for the melts quenched from 1773 to 1823 K were compared to identify the effect of temperature on the unit structures, and these results indicated that Si-OH was more pronounced at low TiOx/SiO2 and less pronounced at high TiOx/SiO2 at higher temperatures. The changes in the structural units as a function of the composition and temperature correlated well with the hydrogen solubility. Basic oxides of MnO additions can depolymerize the melt by providing O2-, which can increase or decrease the hydrogen solubility depending on the dominant hydrogen dissolution mechanism. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.