The microscopic phase behavior of the supercritical carbon dioxide (scCO(2)) + polyethylene oxide-2,6,8-trimethyl-4-nonyl ether (TMN) + water systems at about 3 wt% of TMN were investigated using a synthetic method with a microscope. The two types of TMN (TMN-3 and TMN-10) used in this work had molecular weight distributions caused by the distribution of the number of ethylene oxide groups. Two different types of phase transition were observed when pressure was decreased gradually at a constant temperature from the high pressure at which the transparent phase was observed to the low pressure at which the separate vapor-liquid phases were observed for the scCO(2) + TMN-3 + water system at 3 wt% of TMN-3. The-transparent phase was colorless under all experimental conditions and the phase transition from a transparent phase to a turbidity phase with small, dispersed droplets was observed at the higher side phase transition (higher phase transition pressure). As the pressure continued to decrease, another phase transition was observed from the phase with small droplets to a state with an accelerating aggregation of droplets (lower phase transition pressure). In the turbidity phase between the higher and the lower phase transition, the degree Of turbidity became higher with decreasing pressure. On the other hand, in the phase observed below the lower phase transition, a new liquid phase adhered to the sapphire windows and the wall inside the optical cell. The effect of water concentration on the phase transition pressures for the scCO(2) + TMN-3 + water system was investigated from the results at I wt% and 3 wt% of TMN-3. The higher phase transition pressures at 3 wt% of TMN-3 were lower than those at I wt%, as measured in our previous work. Moreover, the minimal values of the higher phase transition pressure were observed for the system at 3 wt% of TMN-3 as well as those at I wt%. The lower phase transition pressures of the scCO(2) + TMN-3 + water system at 3 wt% of TMN-3 decreased as the water concentration increased to about 1 wt%, and the lower phase transition pressures stayed constant in the range of water concentration from about 1 to 3.62 wt%. On the other hand, the homogeneous phase was not established at water concentrations higher than about 3.86 wt%. As for the scCO(2) + TMN-10 + water system, a transparent homogeneous phase could not be observed in the range of the investigation at 3 wt% of TMN-10. (C) 2009 Elsevier B.V. All rights reserved.