The present invention is related to a method for the synthesis of ethane-1-hydroxy-1,1-diphosphonic acid or its salt which includes the steps of reacting tetraphosphorus hexaoxide and acetic acid under controlled reaction conditions; hydrolyzing the formed ethane-1-hydroxy-1,1-diphosphonic acid condensates to form ethane-1-hydroxy-1,1-diphosphonic acid; further processing the ethane-1-hydroxy-1,1-diphosphonic acid solution. The process according to the method of the present invention is highly controllable and further is characterized by a high selectivity.