The colloidal state (sol, flocs, gels) of Na-montmorillonite dispersions (from bentonite from Bavaria, Germany) was adjusted by the addition of poly(ethylene oxides) with terminal trimethylammonium groups. The colloidal state of the dispersions changed with the molar mass of these end-capped poly(ethylene oxides). The sol-gel diagrams (up to 4.5% (w/w) montmorillonite) in the presence of TMA-PEO 1500 and TMA-PEO 4000 (up to 40 g/l) were characterized by large fields of gel whereas TMA-PEO 20000 and TMA-PEO 35000 caused peptization by steric stabilization. Even dispersions with the highest montmorillonite content (4.5%) were peptized by >0.2 g/l TMA-PEO 35000. The critical coagulation concentration of NaCl was increased by the polymer addition, but the increase was modest (from 8–12 mmol/l to 50 mmol/l). The changes of the colloidal state, the flow behaviour and the salt coagulation reveal the sensitive interplay between stabilization mechanisms (electrostatic, steric) and destabilization effects (bridging, charge compensation, depletion flocculation).