Importance of the 'mixing scale' of reagents is shown through deformation experiments under shear flow on a reactive crosslinked dispersed phase. The EVA dispersed phase in a PDMS matrix can be crosslinked by ester-ester exchange reaction. Tetrapropoxyorthosilicate (TPOS) is used as the crosslinking agent and dibutyltin oxide (DBTO) is used as the precursor. Notion of 'droplet-reactors' is defined. Divergence of the reagents concentration in reactive EVA droplets from the bulk concentration is noticed. Diffusion of the liquid crosslinking agent (TPOS) from the reactive droplets into matrix is strongly inferred, as well as the heterogeneous dispersion of DBTO. First, influence of the mixing time on the dispersion of a powder catalyst (DBTO) in a highly viscous polymer is studied. Second, molecular mixing is experimented through distannoxanes formation when mixing DBTO and EVA. Microscale homogeneous reaction leads to an homogeneous deformation under shear flow of the crosslinked EVA droplets in PDMS matrix, meaning a homogeneous dispersion of the reagents in the EVA. (c) 2005 Elsevier Ltd. All rights reserved.