The rheological and thermomechanical behaviors of toughened epoxy resins filled with thermosetting micro-sized spherical particles were investigated. Two epoxy systems were used to prepare microspheres: a Bisphenol A-type resin (EPON825) cured with 2,4-diaminotoluene and a Bisphenol F-based epoxy resin (PY306) crosslinked with diethyltoluenediamine. Both systems of microspheres were synthesized through dispersion polymerization and differed to each other in size, as evidenced by morphological analysis. These microparticles were blended, in different weight percents (10 and 20 wt%), with a matrix consisting of diglycidyl ether of Bisphenol A (EPON828) and 3,3'-diaminodiphenylsulphone (3,3'DDS). Rheological behavior of the prepared blends was preliminarily studied. After cure, the dynamic-mechanical properties of the composites were also investigated. Results indicated that the reactivity of the uncured blends, as well as the viscoelastic properties of crosslinked systems, are influenced not only by the nature and the amount of the microparticles introduced, but also, in a significant way, by their size.