The effect of polymethylmethacrylate, PMMA, molecular weight on the interfacial morphology in E-glass fibers reinforced epoxy-PMMA matrices has been studied. Four PMMA materials with different molecular weights (34000, 65000, 76000 and 360000 g/mol) were selected. Discrete PMMA rich domains dispersed in a continuous epoxy rich phase were obtained after phase separation in all the samples except when the PMMA molecular weight was 360000 g/mol, where phase inversion occurred. Image analysis was used to obtain a distribution function of PMMA domains. Distributions based on the area occupied by PMMA as a function of the distance from the surface of the glass fiber were obtained from the number of PMMA domains per square micrometer (μm2) of sample. Then, to minimize the experimental noise, cumulative distributions of PMMA areas were used. These data were fitted with a Weibull function. The stabilization distance of the distributions varied with the sample. A distance of approximately 6 μm was found for PMMA 34000 g/mol, whereas for the systems with 65000 and 76000 g/mol this stabilization occurred at longer distances. Results were interpreted in terms of a gradual induced phase separation at the interphases depending on the distance from the glass fiber surface.