Critical polymer concentrations of polyethylene solutions in pentane were measured using a variable-volume view cell apparatus. The molecular weights of the polyethylene samples were 27 700 and 41700 with polydispersity indexes M-w/M-n = 2.00 and 3.70, respectively. The phase separation pressures (cloud point) of the polymer solutions were repeatedly measured in the polymer concentration range 0.3-11.0 mass %. Two types of phase separation patterns (dew-point and bubble-point types) were observed, depending on the polymer concentration. The critical polymer concentration was determined by locating a merging point of the two types of phase separation branches. The critical polymer concentrations for the two polyethylene solutions were 6.90 +/- 0.1 (M-w = 27 700) and 3.95 +/- 0.05 (M-w = 41700) mass %, respectively.