Abstract This paper discusses the influence of the pozzolanic and the mierofiller effects, provided by silica fume, on the microstructure and chemical composition of the transition zone in concretes of low water-cement ratio. In order to characterize these effects separately, carbon black, which has similar size and shape as silica fume, was introduced as a non-reactive mierofiller. SEM observations, porosity and size measurements of the transition zone were carried out. The results demonstrated that carbon black was as effective in filling the spaces in the transition zone, as silica fume. At the early age of one day each of the two materials caused significant densification inside the transition zone. The microstructure had similar appearance in both cases, thus indicating mierofiller effect which is independent of the reactivity of the filler. In time, up to two months, silica fume developed some additional densification, related to the pozzolanic effect. The width of the transition zone, evaluated at one day, did not depend on such factors as water-cement ratio, aggregate size and orientation. Keywords: Transition Zone, Microfillers, Silica Fume, Microstructure.