A long-ranged attractive force was recently detected between two mica plates immersed in a quasibinary polymer solution (Freyssingeas et al. Langmuir 1998,14, 5877-5889). The quasi-binary polymer solution was aqueous ethyl(hydroxyethyl)cellulose (EHEC), where the EHEC had a broad polydispersity. The long-ranged attractive force in the EHEC solution could not be attributed to classical mechanisms such as depletion or bridging. In this study, we investigated if this attractive force can give rise to instability effects in mixed polymer-particle solutions. Accordingly, the effects of added particles on the phase behavior of aqueous EHEC solutions were investigated by cloud point measurements. Aqueous EHEC solutions phase separate on heating. Three different samples of EHEC were investigated, including hydrophobically modified EHEC. As colloidal particles, silica and polystyrene latex were used. The dispersed colloidal particles lowered the cloud point temperature at low polymer concentrations for all EHEC-particle combinations. This particle-induced phase separation is discussed in terms of surface effects.