Ioncell is a Lyocell-based process that utilizes protic superbase-based ionic liquids (ILs) to produce man-made cellulose fibers. This process comprises a dry-jet wet spinning technology, in which dissolved cellulose solutions are stretched in an air gap before regenerating in a water coagulation bath. Previously, 7-methyl-1,5,7-triazabicyclo [4.4.0] dec-5-enium acetate ([mTBDH][OAc]), a guanidine-based IL, and 1,5-diaza-bicyclo[4.3.0]non-5-enium acetate ([DBNH][OAc]), an amidine-based IL, were described as excellent cellulose solvents. This is due to their intrinsic ability to directly dissolve high cellulose concentrations, and produce regenerated fibers of good mechanical properties. However, due to hydrothermal instabilities of the IL superbases (cations), the thermal removal of water from the IL-water mixtures, present in the coagulation bath, can lead to the formation of by-products that may significantly affect the quality and composition of the recycled solvent. The alterations in the solvent composition originate mainly from nonequimolar vaporization of the IL, combined with hydrolysis reactions with water during thermal recovery. Residuals from the superbase synthesis process might also be present. In our previous experience, the dissolution of cellulose is hindered if one of the above-mentioned compounds or their combination rises above certain thresholds. In this study, we investigate the alterations in [mTBDH] [OAc] and [DBNH] [OAc] and correlate them with the ability to completely dissolve cellulose in a concentration of 13 wt %. Moreover, we also show the rheological properties of the prepared solutions and discuss the spinnability limitations of both ILs. The results indicate that [mTBDH] [OAc] tolerates higher alterations from the ideal solvent composition while maintaining its excellent dissolution potential. The same was found for the stability of the fiber spinning process.