The electrophilic nature of chalcones (1,3-diphenylprop-2-en-1-ones) and many other α,β-unsaturated carbonyl compounds is crucial for their biological activity, which is often based on thiol-mediated regulation processes. To better predict their biological activity a simple screening assay for the assessment of the second-order rate constants (k2) in thia-Michael additions was developed. Hence, a clear structure–activity relationship of 16 differentially decorated hydroxy-alkoxychalcones upon addition of cysteamine could be established. Moreover, amongst other naturally occurring α,β-unsaturated carbonyl compounds k2 values for curcumin and cinnamaldehyde were gained while cinnamic acids or esters gave no or very slow reactions.