In this paper we show that dye distributions within polymer core/shell nanoparticles (NPs), grown in multiple steps, are controlled not only by the synthesis route, but are also governed by dye diffusion between core and shell, resulting in dye distributions completely different from the targeted ones. We show that Förster Resonance Energy Transfer (FRET) dynamics can be used as a spectroscopic measure to determine the particular dye profiles along the NPs and thus to uncover the presence of dye diffusion. Finally, to confirm the previous results and with the aim to have a clearer understanding of the diffusion processes within core/shell NPs (equilibrium and transient dye distributions, time scales, etcetera), we analyze the behaviour predicted by existing diffusion models and infer that the dye diffusion is already finished at the end of the shell growth.