The retention of fibre fines and clay particles in chemi-thermo-mechanical pulp (CTMP) suspensions with poly (ethylene oxide) as a retention aid was investigated. The experiments were conducted at 303 and 343 K. These two temperatures correspond to points below and above the Lower Critical Solution Temperature (LCST) of a 0.005 mass% Poly (ethylene oxide) solution in the presence of 2 mol/L KCl. The KCl was added to lower the LCST to a temperature near papermaking conditions. Poly (ethylene oxide) with molar masses of 1 x 10(6) and 5 x 10(6) Da was used. All the LCST measurements of the poly (ethylene oxide)-KCl system agree with those in the literature. The retention study revealed that considerably more fibre fines as well as clay are retained at temperatures above the LCST. In particular, the average fibre fines retention was 5.9 percent at 303 K and 22.1 percent at 343 K when the poly (ethylene oxide) with the molar mass of 1 x 10(6) Da was used. Clay retention was also measured and found to be 5.0 and 15.9% at 303 and 343 K, respectively. Higher retention values were obtained when the polymer with a molar mass of 5 x 10(6) Da was used. At 303 K the fibre fines retention was 55.0%, whereas at 343 K it was 82.2%. The corresponding clay retentions were 31.7 and 71.9%.