The kinetic isotope effect (KIE) method was applied to study the mechanism of elimination of bromine from p-nitro-erythro-a,b-dibromocinnamic acid labeled at the a carbon. This compound was obtained starting from [2-¹⁴C]malonic acid via [2-¹⁴C]cinnamic acid and subsequent addition of bromide. The value of ¹⁴C KIE determined for a-position of side chain of p-nitro-erythro-a,b-dibromocinamic acid proves that elimination of bromine leading to formation of (E)-p-nitrocinnamic acid proceeds via E2 mechanism.