Biotin-PEG7-Maleimide is a biotinylated biochemical assay reagent, which can be used as a thiol-specific probe. Cysteine-linked ADCs prepared with N-aryl maleimides exhibit less than 20% deconjugation in thiol-containing buffer Cysteine-linked ADCs prepared with N-aryl maleimides therefore markedly enhanced stability when compared with analogous ADCs prepared using N-alkyl maleimides, which undergo 35–67% deconjugation under identical conditions. Cysteine-linked ADCs prepared with N-aryl maleimides and incorporating the anticancer drug N-phenyl maleimide monomethyl-auristatin-E (MMAE) maintain strong cytotoxicity following prolonged serum exposure, whereas N-alkyl maleimide MMAE ADCs progressively lose potency.