Blends of an aromatic polyethersulfone (commercial name Victrex) and a polyimide (com- mercial name Matrimid 5218), the condensation product of 3,3’,4,4’-benzophenone tetra- carboxylic dianhydride and 5 (6)-amino-1- (4’-aminophenyl) -1,3,3’-trimethylindane,

were studied by differential scanning calorimetry, dynamic mechanical analysis, and rheological techniques. The blends appeared to be miscible over the whole range of compositions when cast as films or precipitated from solution in a number of solvents. After annealing above the apparent phase boundary, located above Tg,the blends were irreversibly phase separated indicating that the observed phase boundary does not represent a true state of equilibrium. Only a narrow “processing window” was found for blends containing up to 20 wt % polyimide. Rheological measurements in this range of compositions indicated that blending polyether- sulfone with polyimide increases the complex viscosity and the elastic modulus of the blends. For blends containing more than 10 wt % polyimide, abrupt changes in the rheological properties were observed at temperatures above the phase boundary. These changes may be consistent with the formation of a network structure (due to phase separation and/or crosslinking) .Blends containing less than 10 wt % polyimide exhibited stable rheological properties after heating at 320°C for 20 min, indicating the existence of thermodynamic equilibrium.