We have developed a conjugation reaction based on the thia-Michael addition to activated triple bonds, which can be an alternative to maleimides, the most commonly used reagents to link thiol groups (of Cys) to drugs and labels. An amino group is converted into its propynamide and, in aqueous media at 37 °C and pH 7.4, Cys derivatives are added. The oxopropene-1,3-diyl linker is formed with excellent Z selectivity without secondary reactions. No exchange with other thiols is observed.