AbstractThe addition of trimethylsilyl esters of trivalent phosphorus acids to trimethylsilyl ester of 1 H‐indene‐3‐carboxylic acid has been studied. The convenient syntheses of new functionalized organophosphorus acids and their derivatives with indan‐1‐carboxylic acid moieties have been proposed. Water‐soluble sodium salts of the corresponding acids were also obtained.