We present new experimental measurements of the molecular weight (MW) distributions in coexisting liquid phases for three polymer/solvent systems. We studied samples at the critical compositions and at four temperatures in the two-phase regions. For polystyrene in methylcyclohexane (with an upper critical solution point), we observed the expected fractionation between phases, with a higher average MW in the lower, polymer-rich phase. For poly(ethylene oxide) in 2, 6-lutidine+water (with a lower critical solution point), the total polymer mass is nearly equal in the two phases, but the polymers of higher average MW equilibrate into the upper, 2,6-lutidine-rich phase. For poly(ethylene oxide) in isobutyric acid+water (with an upper critical solution point), most of the total polymer mass is in the upper, isobutyric acid-rich phase, but the polymers of higher average MW equilibrate into the lower, water-rich phase; moreover, the fractionation in this case is quite dramatic, with almost a factor of 2 difference in average MW between the phases and a significant decrease in polydispersity for the lower phase. Our results support prior experimental results on this phenomenon that find that a basic Flory-Huggins (FH) theory is inadequate. More recent theoretical considerations suggest that the difference between FH theory and experiments is due to non-mean field effects, but this theory awaits direct comparison to experiments.