Deep mantle melts contain massive amounts of CO2 but three critical issues related to the effect of CO2 on molecular structure and physical properties of magmatic melts remain poorly constrained: 1) there is no exact picture of CO2 dissolution mechanism in silicate melt, 2) the silicate melt polymerization upon CO2 dissolution has not been reliably quantified and 3) the effect of CO2 on silicate melt viscosity has never been measured adequately.We synthesized a series of 17O and 29Si-enriched CO2-bearing melilitite (SiO2 ~ 35 wt%) silicate glasses at high temperature and pressure. Using NMR spectroscopy, we have interrogated both anionic and cationic networks for quantifying the change in the degree of polymerization associated to CO2 incorporation in the melt. Increasing CO2 content induces a strong increase in the degree of polymerization. CO2 dissolution follows a complex mechanism involving the formation of Free Ionic Carbonate (FIC) Ca2 +·CO32– species. This carbonate subnetwork is the precursor to the immiscibility process between a carbonate liquid and a silicate liquid.Glass transition temperature (Tg) measurements show that increasing CO2 content induces a decrease in Tg implying a decrease in viscosity for the studied low silica melt composition. This result appears in complete contradiction with the melt polymerization induced by CO2 as quantified by NMR. We propose a model that reconciles both aspects. CO2 induces silicate subnetwork polymerization resulting in a viscosity increase but it also induces a competing effect by forming a carbonate subnetwork having a low viscosity. The overall result appears dominated by the carbonate subnetwork resulting in a slight decrease in melt viscosity in agreement with existing studies.