The effect of sulfur dissolved in anorthite-diopside eutectic (AD) glasses on the glasstransition temperature (Tg) has been investigated via Differential Scanning Calorimetricmeasurements (DSC) and Thermogravimetric Analysis (TGA) under moderately reducing tooxidizing conditions.In a series of AD glasses, we have measured the change in Tg as a function of S contentpresent as SO42- (HS- is also identified to a lesser extent) and H2O content. The AD glassesinvestigated have S contents ranging from 0 to 7519 ppm and H2O contents ranging from 0 to5.3 wt.%. In agreement with previous studies, increasing H2O content induces a strongexponential decrease in Tg: volatile free AD glass has a Tg at 758±13C and AD glass with5.18±0.48 wt.% H2O has a Tg at 450±11C. The change in Tg as a function of H2O is wellreproducedwith a third-order polynomial function and has been used to constrain Tg at anyH2O content. The effect of S on Tg is almost inexistent or towards a decrease in Tg withincreasing S content. For instance, at ~2.4 wt.% H2O, the addition of S induces a change inTg from 585±10°C with 0 ppm S to 523±3C with 2365±138 ppm S; a further increase in Sup to 7239±90 ppm S does not induce a dramatic change in Tg measured at 529±2C.The limited effect of S on the glass transition temperature contrasts with recent spectroscopicmeasurements suggesting that S dissolution as SO42- groups provokes an increase in thepolymerization degree. We propose an alternative view which reconciles the spectroscopicevidence with the Tg measurements. The dissolution of S as SO42- does not induce theformation of Si-O-Si molecular bonding through consumption of available non-bridgingoxygens (NBO) but instead we suggest that Si-O-S molecular bonds are formed which are not detectable by DSC measurements but mimic the increase in glass polymerization. Therefore, spectroscopic measurements must be used with caution in order to extract melt physicalproperties.