Covalent modifications of thiol and amine groups may control the function of proteins involved in the regulatory and signaling pathways of the cell. In this study, we developed a simple cysteamine assay which can be used to study the reactivity of electrophilic compounds towards primary amine and thiol groups in an aqueous environment. The detection principle is based on the electrochemical, photometrical and mass spectrometric analyses of cysteamine (2-aminoethanethiol) as the molecular probe. This technique is useful for studying the reaction kinetics of electrophiles with thiol (SH) and amino (NH2) groups. The decrease in analytical responses of cysteamine was monitored to evaluate the reactivity of three electrophilic activators of the Nrf2 pathway, which mediates the cellular stress response. The SH-reactivity under cell-free conditions of the tested electrophiles decreased in the following order: 4-hydroxy-2-nonenal ≥ nitro-oleic acid > sulforaphane. However, as shown in RAW264.7 cells, the tested compounds activated Nrf2-dependent gene expression in the opposite order: sulforaphane > nitro-oleic acid ≥ 4-hydroxy-2-nonenal. Although other factors in addition to chemical reactivity play a role in biological systems, we conclude that this cysteamine assay is a useful tool for screening potentially bioactive electrophiles and for studying their reactivity at a molecular level.