A method for the synthesis of alkane-1-hydroxy-1,1-diphosphonic acid or its salts, under controlled thermal conditions, by substantially avoiding the formation of phosphines and low oxides of phosphorus and the accumulation of phosphite intermediates, comprising the steps of: a) forming a reaction mixture by: - gradually adding tetraphosphorus hexaoxide, under optimal mixing conditions, to the liquid phase and below the surface of the liquid phase comprising a carboxylic acid, while controlling and maintaining the temperature of the reaction mixture at a preset fixed value in the range of from 80°C to 200°C, said carboxylic acid having an atmospheric boiling point of 100°C or more above the temperature of the reaction mixture or - gradually injecting tetraphosphorus hexaoxide, under optimal mixing conditions, below the surface of the liquid phase comprising a carboxylic acid, while controlling and maintaining the temperature of the reaction mixture at a preset fixed value in the range of from 80°C to 200°C, said carboxylic acid having an atmospheric boiling point of 40°C or less, preferably of 60°C or less, more preferably of 80°C or less, most particularly of less than 100°C above the temperature of the reaction mixture, wherein said reaction mixture comprises a molar ratio of carboxylic acid to tetraphosphorus hexaoxide comprised between 3 and 15, preferably between 4 and 10, more preferably between 4 and 8, selected in such a way that the viscosity of the reaction mixture is 10 Pa.s or less, preferably 8 Pa.s or less, more preferably 6 Pa.s or less and most preferably 5 Pa.s or less, as measured by an in process vibrational viscometer calibrated with Certified Standard Oils, enabling said optimal mixing conditions, the gradual addition or injection leading to the formation of alkane-1-hydroxy-1,1-diphosphonic acid condensates; b) hydrolyzing the alkane-1-hydroxy-1,1 -diphosphonic acid condensates of the reaction mixture of step a) by adding water, wherein the molar ratio of water to tetraphosphorus hexaoxide, added in step a) is 1 or more, and maintaining the reaction mixture at a temperature of at least 100°C to obtain a solution comprising the alkane-1-hydroxy-1,1-diphosphonic acid. a) forming a reaction mixture by: - gradually adding tetraphosphorus hexaoxide, under optimal mixing conditions, to the liquid phase and below the surface of the liquid phase comprising a carboxylic acid, while controlling and maintaining the temperature of the reaction mixture at a preset fixed value in the range of from 80°C to 200°C, said carboxylic acid having an atmospheric boiling point of 100°C or more above the temperature of the reaction mixture or - gradually injecting tetraphosphorus hexaoxide, under optimal mixing conditions, below the surface of the liquid phase comprising a carboxylic acid, while controlling and maintaining the temperature of the reaction mixture at a preset fixed value in the range of from 80°C to 200°C, said carboxylic acid having an atmospheric boiling point of 40°C or less, preferably of 60°C or less, more preferably of 80°C or less, most particularly of less than 100°C above the temperature of the reaction mixture, wherein said reaction mixture comprises a molar ratio of carboxylic acid to tetraphosphorus hexaoxide comprised between 3 and 15, preferably between 4 and 10, more preferably between 4 and 8, selected in such a way that the viscosity of the reaction mixture is 10 Pa.s or less, preferably 8 Pa.s or less, more preferably 6 Pa.s or less and most preferably 5 Pa.s or less, as measured by an in process vibrational viscometer calibrated with Certified Standard Oils, enabling said optimal mixing conditions, the gradual addition or injection leading to the formation of alkane-1-hydroxy-1,1-diphosphonic acid condensates; - gradually adding tetraphosphorus hexaoxide, under optimal mixing conditions, to the liquid phase and below the surface of the liquid phase comprising a carboxylic acid, while controlling and maintaining the temperature of the reaction mixture at a preset fixed value in the range of from 80°C to 200°C, said carboxylic acid having an atmospheric boiling point of 100°C or more above the temperature of the reaction mixture or - gradually injecting tetraphosphorus hexaoxide, under optimal mixing conditions, below the surface of the liquid phase comprising a carboxylic acid, while controlling and maintaining the temperature of the reaction mixture at a preset fixed value in the range of from 80°C to 200°C, said carboxylic acid having an atmospheric boiling point of 40°C or less, preferably of 60°C or less, more preferably of 80°C or less, most particularly of less than 100°C above the temperature of the reaction mixture, b) hydrolyzing the alkane-1-hydroxy-1,1 -diphosphonic acid condensates of the reaction mixture of step a) by adding water, wherein the molar ratio of water to tetraphosphorus hexaoxide, added in step a) is 1 or more, and maintaining the reaction mixture at a temperature of at least 100°C to obtain a solution comprising the alkane-1-hydroxy-1,1-diphosphonic acid.