We prepared a stable and highly viscous suspension of cellulose microcrystals by postsulfating the HCl-hydrolyzed cellulose microcrystals. The neutralized suspension was isotropic and showed flow birefringence at low solid content (ca. <1%). At a solid content of approximately 2-3%, the suspension was still flowing but showed intricate birefringent patterns, which remained after the flow had ceased. At high solid content (7.1%), the suspension became too viscous to flow and showed a frozen-in birefringent pattern with a microscopic crosshatch texture. These features show that this postsulfated suspension forms a "birefringent glassy phase", similar to that observed for a boehmite rod suspension, being distinctly different from the behavior of the suspension prepared by direct hydrolysis of cellulose with concentrated sulfuric acid.