Polymeric dispersants that adsorb onto various types of fine particles have been designed through complex formation between cationic polyethyleneimine (PEI) and anionic oleic acid (OA) to improve particle surface compatibility with nonaqueous solvents. Complex formation involves the OA-assisted dissolution of PEI in a nonaqueous solvent (toluene) by ultrasonication. While PEI itself is immiscible in toluene, various molecular weights (MWs) of PEI (M-W = 600, 1800, and 10 000) visibly dissolved when, in the presence of more than 5-mol % PEI (based on the ethyleneimine (El) unit of PEI), a complex formed between the carboxyl group of OA and the amines of PEI in toluene. Si3N4 and one of the typical multicomponent compositions for Si3N4 ceramic fabrication, Si3N4-Y2O3 MgO, were chosen to demonstrate the usage of PEI OA complexes for improving nonaqueous slurry stability. While nontreated Si3N4 particles rapidly formed dense slurry aggregates followed by solidification in toluene, PEI associated with 30 mol % OA effectively adsorbed on Si3N4 fine particles and drastically improved the flow properties with similar to 1.8 mg/m(2) treatment. Surprisingly, the flow properties were maintained even for dried and redispersed slurries. The effect of the MW of PEI OA and its additive content on the stability of Si3N4/toluene slurries has been clarified, and their applicability toward Si3N4-Y2O3-MgO multicomponent systems is discussed.