Nonyl 4-Hydroxybenzoate-d4 is labelled Nonyl 4-Hydroxybenzoate which was used to prepare 4-[(aminosulfonyl)oxy] benzoic acid derivatives as potent inhibitors of the enzyme estrone sulfatase (ES). It was also used to synthesize sulfonylbenzoic acid esters as potential inhibitors of oestrone sulphatase with antitumor activities.