The glass transition and the structural relaxation processes of blends of polystyrene and poly(2,6-dimethyl-1,4-phenylene oxide) have been studied by differential scanning calorimetry. The experimental results were compared with the prediction of a model based on the calculation of the configurational entropy of the sample during the thermal history. The model is fitted to the experimental results, and material parameters independent of the thermal history are obtained. These parameters are used to estimate the length of the cooperativity unit at the glass-transition temperature for each blend and pure polymer.