We have investigated the specific features of the steady shear flow of concentrated spinning solutions of cellulose and its mixtures with polyvinyl alcohol, chitosan, and polyacrylonitryl in orthophosphoric acid in the temperature range of 303–323 K. Rheological curves including highest- and lowest-viscosity regions have been obtained. The influence of the nature of the polymer additive and its content on the profile of rheological curves of the cellulose solution has been established.