As a weak base, beta-glycerophosphate (beta-GP) was used to spontaneously initiate gelation of quaternized cellulose (QC) solutions at body temperature. The QC/beta-GP solutions are flowable below or at room temperature but gel rapidly under physiological conditions. In order to clarify the sol gel transition process of the QC/beta-GP systems, the complex was investigated by dynamic viscoelastic measurements. The shear storage modulus (G') and loss modulus (G '') as a function of (1) concentration of beta-GP (c(beta-Gp)), (2) concentration of QC (c(QC)), (3) degree of substitution (DS; i.e., the average number of substituted hydroxyl groups in the anhydroglucose unit) of QC, (4) viscosity-average molecular weight (M-eta) of QC, and (5) solvent medium were studied by the oscillatory rheology. The sol-gel transition temperature of QC/beta-GP solutions decreased with an increase of c(QC) and c(beta-GP), the M-eta of QC, and a decrease of the DS of QC and pH of the solvent. The sol-gel transition temperature and time could be easily controlled by adjusting the concentrations of QC and beta-GP, M-eta and DS of QC, and the solvent medium. Gels formed after heating were irreversible; i.e., after cooling to lower temperature they could not be dissolved to become liquid again. The aggregation and entanglement of QC chains, electrostatic interaction, and hydrogen bonding between QC and beta-GP were the main factors responsible for the irreversible sol-gel transition behavior of QC/beta-GP systems.