Orthophosphate reacts with molybdate in acidic medium to yield 12-molybdophosphoric heteropolyacid; subsequently, detection is undertaken either on the molybdophosphate reduction product (molybdenum blue method) or on the yellow vanadomolybdate complex [2,5,6]. The molybdenum blue method became broadly used due to its higher sensitivity and lower susceptibility to interferences [4,7,8], than the vanadophosphomolybdate method. Nevertheless, the low concentration levels imposed by legislation are difficult to achieve. Therefore, the development of new methodologies capable of monitoring trace phosphate levels in environmental samples with adequate precision, has gained importance [9]. Flow injection analysis (FIA) is a widespread and well established approach to determine phosphorus. In fact, the first publication on FIA is dedicated to phosphate determination [10] and discusses both the yellow vanadomolybdate and the molybdenum blue method. Compared to the blue molybdenum method, the yellow vanadomolybdate analytical procedure can present some advantages that make it more suitable for flow systems: (i) the use of a single reagent mixture with extended stability makes the method suitable for field portable applications and for unattended operation in water monitoring [11,12] and (ii) the formation of a fine precipitate, accompanying the development of the blue colour, does not occur. However, maybe because of its limited sensitiv