Detailed proton NMR examination of transreacting poly(2-ethyl-2-methylpropylene terephthalate) (PEMPT)/Bisphenol-A polycarbonate (PC) 50/50 (w/w) blends enables the simultaneous monitoring of both alcoholysis and midchain transreactions. In the absence of midchain reaction, alcoholysis via the hydroxypropylene end group of PEMPT and the carbonate group of PC enhances miscibility over a range of molecular weights. At the transition point from two phase to single phase, 5.7 mol % of the PEMPT repeat units are connected to PC via alcoholysis. In a PEMPT/PC blend of high molecular weight undergoing simultaneous alcoholysis and midchain reaction, the extent of reaction required to shift the phase behavior from two phase to one phase is 4%. This corresponds to 2.8% alcoholysis and 1.2% midchain transreaction. These levels of reaction required for phase transition are in theoretical agreement with simple models of the reacting blend.