The rheology and microstructure of two different cellulose nanocrystals (CNC) samples possessing different degrees of sulfation are studied over a broad concentration range of 1 to 15 wt%. CNC suspensions are isotropic at low concentration and experience two different transitions as concentration increases. First, they form chiral nematic liquid crystals above a first critical concentration where the samples exhibit a fingerprint texture and the viscosity profile shows a three-region behavior, typical of liquid crystals. By further increasing the concentration, CNC suspensions form gels above a second critical concentration, where the viscosity profile shows a single shear-thinning behavior over the whole range of shear rates investigated. It has been found that the degree of sulfation of CNC particles has a significant effect on the critical concentrations at which transitions from isotropic to liquid crystal and liquid crystal to gel occur. Rheological properties and microstructure of these suspensions have been studied using polarized optical microscopy combined with rheometry.