tert.‐Butyl amino carbonate (1,tert.‐butyloxycarbonyloxyamine) was prepared by reaction of hydroxylamine with di‐tert.‐butyl dicarbonate (2). 1 was used to acylate different amino acids or amines in water or in aqueous dioxane. 1 was also preparedin situand used to acylate amino acids directly. 1 reacted 1.5–2.5 times faster than 2 with all amines studied either in water or 50% (v/v) dioxane. Remarkably, 1 retained its ability to acylate amines even in acid solution; the rate of acylation of L‐Phe at pH 6.5 (15 M‐1˙ min‐1) was about 20% of the rate at pH 10 (72 M‐1˙ min‐1). 2 was not an acylating reagent below pH 7.0 and as expected, the rate at pH 8.3 (4 M‐1˙ min‐1) was about 10% of that at pH 9.3 (39 M‐1˙ min‐1). Pure BOC‐amino acids were obtained in high yield and could be quantitatively d