Water uptake phenomena was studied in a group of commercial epoxy based thermosets using gravimetric and fluorimetric analysis. The different epoxy formulations were labeled with two dansyl derivatives differing in the spacer length between the chain and the fluorophore moiety. The fluorimetric method consisted of monitoring the changes in the first moment of the dansyl emission band as a function of water immersion time. Using the fluorescence, it was possible to obtain the parameters that govern the water diffusion process and there was a good concordance with gravimetric results. Furthermore, the fluorescence response of the dansyl moieties was used to study the effect of the molecular structure of the polymers in the water absorption process.