Two-step catalytic waiting time ($p(t)=\frac{k_a{k}_b}{k_a-k_b}(e^{-k_{b}t}-e^{-k_{a}t})$)

The convolution of two independent sequential exponential waiting times has the displayed normalized density and mean $k_a^{-1}+k_b^{-1}$. At equal rates its limit is $k^{2}t{e}^{-kt}$. A measured density of this form supports a multistep kinetic mechanism without uniquely identifying its hidden chemical intermediates.