Additives that alter the rheology of lignocellulosic biomass suspensions were tested under conditions of variable pH, temperature, and solid concentration. The effects of certain ions, biomass type, and time after the addition of rheological modifier were also examined. Torque and vane rheometry were used to measure the yield stress of samples. It was found that the effectiveness of rheological modifiers depends on pH over a range of 1.5 to 6, biomass type, concentration of certain ions, and time after addition. The time-dependent properties of rheologically modified biomass are sensitive to the type of rheological modifier, and also to mixtures of these additives, which can result in unexpected behavior. We show that time-dependent rheology is not correlated with time-dependent changes of the water-soluble polymer (WSP) in the aqueous environment, such as slow polymer hydration, suggesting that time-dependent changes in the polymer-fiber interaction may play a more significant role.