The small angle neutron scattering (SANS) technique has been used to study the interfacial structure development in poly(methyl methacrylate)/polycarbonate (PMMA/PC) blends through transesterification. We found : 1) Development of interfacial structure through transesterification at the phase separated domain interfaces are observed at T less than or equal to 200 degrees C. At T greater than or equal to 215 degrees C, because of a higher transesterification rate, the melting of domains is observed together with very weak scattering by interfacial structure. 2) Interfacial thickness t(i) increases with time and reaches a plateau value of t(i) at 32.5 +/- 2.4 Angstrom and 29.6 +/- 3.7 Angstrom for 180 degrees C and 200 degrees C, respectively. 3) The intensity profiles at 190 degrees C and long times show the formation of spherical domains similar to those reported by Rabeony et at (1). These may be caused by the crosslinking effect of the transesterification reaction between PMMA and PC.