The simultaneous adsorption of three poly(ethylene oxide) (PEO) with different molecular weights and two cationic surfactants on silica under a constant feed concentration of PEO has been investigated in aqueous solutions. Two cationic surfactants were used; dodecyltrimethylammonium bromide (DTAB) and 1,2-bis(dodecyltrimethyl ammonio) ethane dibromide (2RenQ). The amount of PEO adsorbed decreased with increasing surfactant concentration, while that of the surfactant increased with surfactant concentration but was lower in the presence of PEO than in the absence of PEG, indicating a competitive adsorption. The reduction in PEO adsorption was greater in the presence of 2RenQ than in the presence of DTAB accompanying an increase of the molecular weight of PEG. The dispersion stability of silica suspensions by the competitive adsorption showed a dispersion-flocculation-redispersion sequence with the surfactant concentration, suggesting the importance of electrostatic interactions as well as steric interactions. ESR measurements using spin-labeled PEO show that PEO molecules adsorbed take mainly loops or tails, inducing steric hindrance for the high dispersion stability of silica suspensions.