The methodologies for determining orthophosphate ions by spectrophotometry and flow-injection analysis for 1995–2011 in different features are briefly described in the paper. The method of the spectrophotometric determination of orthophosphate in the form of molybdophosphoric heteropolyacid via reduced ascorbic acid in the presence of potassium antimonyl tartrate has been modified. The method allows analysts to determine 0.1–1.0 mg/L of orthophosphate ions in natural waters and has been tested by analyzing a model solution similar to the subterraneous waters of the Arigol licensed area. The advantages of the developed method, compared to the certified method, consist of the possibility of the express assessment and lower consumption of chemical reagents. The method is suitable for indicator investigations in geophysics.