The so-called molybdenum blue method for assaying the inorganic phosphate concentration of solutions is well established in scientific circles. This study was concerned with what, specifically, is being measured by the molybdenum blue method and whether or not the method is providing valid estimates of the dissolved inorganic phosphate concentration of natural waters. It is concluded that neither a hidden blank error or the hydrolysis of organic phosphate esters will seriously bias the results obtained by the molybdenum blue method. Except in instances where arsenate is a problem the molybdenum reactive material present in natural waters may be regarded as inorganic phosphate, although not necessarily as free phosphate ion.