Aminooxy-biotin can be used to covalently attach biotin to a molecule with an aldehyde or ketone group. The aminooxy group reacts with an aldehyde or ketone group to form a stable oxime linkage in an aqueous solution at neutral pH. The reaction is fast and can be further accelerated by adding aniline as a catalyst. Thus, aminooxy reagents are superior to hydrazide reagents, which also react with aldehydes or ketones but form unstable hydrazone linkages.