The effect of addition of a polyelectrolyte, a nonionic surfactant; ABA block copolymers, and mixtures of the polyelectrolyte with the block copolymers on the theology of coal/water suspensions was studied using steady state and oscillatory theological measurements. The polyelectrolyte was sodium lignosulfonate (Ufoxane 3A). The nonionic surfactant (EL1602P) was hexamethylenediamine with 4 tails of 10 propylene oxide (PO) and 55 ethylene oxide (EO) units. The block copolymers (Synperonic PE) consisted of 55 PO units and two tails of 4-147 EO units per chain. The addition of the polyelectrolyte caused a rapid reduction in the complex modulus (G*) above similar to 0.1% on the basis of the coal. This indicated deflocculation of the suspension above this concentration. This deflocculation was caused by high adsorption of the polyelectrolyte that was accompanied by an increase in the negative zeta potential. The deflocculation was also reflected in the sedimentation behavior of the suspension which showed a decrease in sediment volume at the same concentration at which deflocculation became substantial. The results obtained using the nonionic surfactant were significantly different from those using the polyelectrolyte. They showed an initial increase in the modulus, yield value, and viscosity as the concentration of the surfactant was increased. A maximum was reached at a critical concentration, above which there was a rapid reduction in G* and viscosity. These results were explained in terms of the adsorption characteristics of the surfactant. Initially, the surfactant adsorbs with the PEO chains pointing toward the solid, and this causes flocculation by hydrophobic interaction. At higher surfactant concentration, adsorption occurs via the hydrophobic groups, leaving the PEO tails dangling in solution, and this leads to restabilization. The PEO-PPO-PEO block copolymers showed a gradual decrease in flocculation with an increase in PEO chain length. This was attributed to the increase in adsorbed layer thickness with an increase in PEO chain length. Energy-distance curves show an attractive minimum whose depth becomes smaller as the adsorbed layer thickness increases. Addition of the PEO-PPO-PEO block copolymers to coal suspensions stabilized by the polyelectrolyte showed a small effect when the PEO chain length was <37 units. However, with the largest PEO chain studied (147 units per chain), the addition of the block copolymer caused an initial increase in flocculation (accompanied by a rapid increase in G*), reaching a maximum at an optimum concentration, after which there was restabilization of the suspension. These results were explained in terms of the orientation of the molecule. Initially, the molecule probably adsorbs with the PEO chain toward the surface (on the hydrophilic batches), resulting in flocculation by screening the charge and possible hydrophobic interaction. At higher concentration a second layer is produced with the PEO chains dangling in solution, resulting in restabilization of the suspension. These mixtures of polyelectrolytes and nonionic block. copolymers may find application in the preparation of stable coal/water suspensions.