In the present work, a solid phase extraction (SPE) is hyphenated with an automatic MSFIA system to improve the selenite determination based on the reaction of selenite with aromatic o-diamines (such as 2,3-diaminonaphthalene (DAN)) to form the piazselenol complex. This reaction is greatly influenced by acid concentration, temperature, the time needed for colour development, and presence of foreign ions. For these reasons a thermostatic bath, glycine, and Na2-EDTA are used as heater, buffer, and masking agent, respectively. The principle of the determination is based on the sorption of the piazselenol onto a C18 membrane disk, followed by its elution by acetonitrile. The piazselenol can then be detected by absorptiometry or fluorometry, both detection techniques being tested in our system. The best detection limit (1.7 g L−1) and RSD (3.04%) are obtained by absorptiometry at 380 nm. Environmental samples were spiked and analyzed, with recoveries close to 100%.