Diversification of liquid fuel sources for transportation plays a key role in the energetic transition. High quality and clean diesels can be produced by Fischer-Tropsch synthesis, which interest is renewed in as much it can be performed from syngas (CO + H-2) produced by conversion of natural gas or gasification of lignocellulosic biomass, leading, in this latter case, to fuels from renewable resource. Catalytic performance (activity and selectivity) of cobalt-based catalysts are related to the physico-chemical characteristics of the cobalt nanoparticles. The present study proposes a new approach to obtain a detailed and exhaustive description of the cobalt active phase in FT catalysts. We combined Anomalous Small X ray Scattering (ASAXS) and electron tomography, giving complementary insights on the microstructure and size distribution of both cobalt nanoparticles and aggregates. This approach, carried out on a cobalt/alumina-silica catalysts studied first at the oxide state and then at the reduced state, allowed to highlight the mechanisms involved at the nanoscale during the reduction step. Reduction impact is significant on aggregates morphology, causing their fragmentation and increasing their accessibility. As a perspective, a better knowledge of cobalt aggregates morphology will help to understand their impact on the catalyst performance. (C) 2018 Elsevier Inc. All rights reserved.