method for determining the phosphorus content of an aqueous sample , in particular drainage - water sample , wherein the sample is subjected to thermal - oxidative decomposition and the orthophosphate content of the decomposed sample is determined photometrically in an aqueous analysis solution , such that the thermal decomposition is carried out in a single step by catalyzer - free burning of the sample as batch - decomposition in a combustion oven , the resulting combustion gas is transferred out of the combustion oven in a carrier - gas stream and the combustion / carrier gas stream is cooled to obtain the aqueous analysis solution as a condensate therefrom .