This paper describes the discovery, synthesis, and use of novel water-soluble acylation reagents for efficient and selective modification, cross-linking, and labeling of proteins and peptides, as well as for their use in the effective modification of sepharose beads under pH control in aqueous media. The reagents are based on a 2,4-dichloro-6-sulfonic acid phenol ester core combined with a variety of linker structures. The combination of these motifs leads to an ideal balance between hydrolytic stability and reactivity. At high pH, good to excellent conversions (up to 95%) and regioselectivity (up to 99:1 Nϵ/Nα amine ratio) in the acylation were realized, exemplified by the chemical modification of incretin peptides and insulin. At neutral pH, an unusually high preference toward the N-terminal phenylalanine in an insulin derivative was observed (>99:1 Nα/Nϵ), which is up until now unprecedented in the literature for more elaborate reagents. In addition, the unusually high hydrolytic stability of these reagents and their ability to efficiently react at low concentrations (28 μM or 0.1 mg/mL) are exemplified with a hydroxy linker-based reagent and are a unique feature of this work.