The fracture surface topography of simultaneous interpenetrating polymer networks (SIN’S) of poly(2,6-dimethyl-1,4 phenylene oxide) (PPO)/poly(butadiene) diol was examined by scanning electron microscopy (SEM). The phase morphology of the IPNs is highly dependent on PPO/poly(butadiene) diol mass ratio. A single-phase morphology of the IPNs was achieved when the content of poly(butadiene)diol is below 40 wt % in the initial composition. The SEM observations are also wholly consistent with the glass transition temperature behavior obtained in the differential scanning calorimetric (DSC) measurements.