A closure at the third‐order moment is presented for the problem of the decay of reactants which obey a second‐order equation, and whose initial description is given stochastically. The closure satisfies prescribed realizability conditions for all possible initial assignments of the mean, the mean square fluctuations, and third‐order moments of the concentration field. The closure is applied in two typical examples, and in each case the results agree satisfactorily with exact stochastic solutions.