The adsorption of polyethylene oxide (PEG) on chemi-thermo-mechanical pulp (CTMP), clay, and chalk suspensions was investigated at 303 and 343 K. These conditions corresponded to points below and above the critical solution temperature (CST) of a 0.005 mass % polyethylene oxide solution in the presence of 2 mol/L of KCI. The PEO adsorption on CTMP and clay particles was also studied at different initial PEO concentrations up to 50 mg/L. PEG adsorbed onto papermaking furnish particles at both temperatures, but the adsorbed amount was found to be larger above the CST in all systems. At 303 K it increased with the increase in PEG added to the system until it reached a plateau, but at 343 K it increased with the increase in PEO added in all the range of PEG concentrations up to 50 mg/L. The state of aggregation of PEO-clay and PEG-chalk suspensions was also studied by monitoring fluctuations in the intensity of light transmitted through the suspension. These measurements indicated a strongly temperature-dependent aggregation. It was concluded that the entropically driven phase separation leads to enhanced aggregation, which in film favors the retention of fiber fines and clay filler.