Phase separation in blends of bisphenol A polycarbonate and poly(methyl methacrylate), PC/PMMA, is investigated on the microscopic scale by means of atomic force microscopy (AFM). This technique allows the visualization of the early stages of phase separation with greater accuracy, relative to optical techniques. In comparison to previous data, the AFM-determined demixion vs composition curve appears to be shifted to lower temperatures. Starting from homogeneous thin films, we then follow the thermally induced spinodal decomposition process of 50/50 blends and characterize the morphological changes as a function of demixion time and annealing temperature. We present a quantitative investigation of the growth of the dispersed phase, based on a statistical power spectral density analysis of the AFM data. The interest of this method is to provide information on the growth mechanism, by establishing the scaling relationships between the topographic roughness (which is due to phase separation), the length scale of observation, and the annealing time. In the present case, the phase separation process appears to follow the Kardar-Parisi-Zhang universality class of growth, in which the density is not a conserved quantity. The corresponding morphology is accordingly marked by a clear topographic discontinuity between the PMMA-rich domains and the PC-rich matrix. We also observe that, at temperatures exceeding 220 degreesC, the late stages of the spinodal decomposition process are strongly affected by the occurrence of chemical reactions between PC and PMMA, which slow the growth rate of the dispersed phase and the surface roughening.