To investigate mass transfer into droplets in reactive extraction, a nonintrusive method for measuring concentration gradients using fluorescent dyes as sensors was developed. An optical probe is used to focus a laser beam into the droplet and to detect the fluorescent light. Scanning the droplet with the probe leads to concentration profiles. The added dyes can either be indicators to measure gradients of ionic components in the aqueous phase or be one component of the extraction system. For the description of the system ZnSO4-H2SO4/di(2-ethylhexyl)phosphoric acid in isododecane, experiments have been carried out in the stagnant as well as in the flowing continuous phase. To investigate the effects of internal circulation in the droplets, further mass transfer experiments in a venturi tube have been carried out. A comparison enables both diffusion and convection inside and outside of a droplet in a reactive system to be described.