Phase separation temperature estimations, based on Hansen solubility parameters for poly(ethersulfone) (PES)/ solvent/non-solvent systems, were carried out to study the control of phase separation temperature in a reverse thermally induced phase separation (RTIPS) process. Four membrane-forming systems were studied, namely PES/N, N-dimethylacetamide (DMAc)/diethylene glycol (DEG), PES/DMAc/polyethylene glycol 200 (PEG200), PES/DMAc/PEG300 and PES/DMAc/PEG400. The effects of PES molecular weights, PES concentrations, PEG molecular weights and ratios of non-solvent/solvent on phase separation temperature are investigated, and the theoretical Hansen solubility parameter calculation is used to establish a prediction equation for phase separation temperature. A linear relationship between the experimental data and the difference in the solubility parameters between PES and the mixed solvent was observed. When the membrane-forming temperature was higher than the cloud point, membranes with a bi-continuous structure were acquired and showed a higher pure water permeation flux than that of membranes prepared with the non-solvent induced phase separation (NIPS) process. The pure water permeation flux and the mean pore size of membranes prepared with the RTIPS process decreased in line with an increase of PES molecular weight. When the membrane formation mechanism was the RTIPS process, the mechanical properties were better than those of the corresponding membranes prepared with the NIPS process.