The relaxation kinetics of the diffusion-influenced reversible reaction $A+B\ensuremath{\rightleftharpoons}C$ is studied in the pseudo-first-order limit $([B]\ensuremath{\gg}[A])$ when A and C are static and the B's move independently with diffusion coefficient D. For the initial condition $[A(0)]\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$, $[C(0)]\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$, it is shown that the asymptotics of $[A(t)]$ for $t\ensuremath{\rightarrow}\ensuremath{\infty}$ is given in d dimensions by $({1+K}_{\mathrm{eq}}[B]{)}^{\ensuremath{-}1}{+K}_{\mathrm{eq}}^{2}[B]/({1+K}_{\mathrm{eq}}[B]{)}^{3}{f}_{d}\left(t\right)$ with ${f}_{1}\left(t\right)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(\ensuremath{\pi}\mathrm{Dt}{)}^{\ensuremath{-}1/2}$, ${f}_{2}\left(t\right)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(4\ensuremath{\pi}\mathrm{Dt}{)}^{\ensuremath{-}1}$, and ${f}_{3}\left(t\right)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(4\ensuremath{\pi}\mathrm{Dt}{)}^{\ensuremath{-}3/2}$, and where ${K}_{\mathrm{eq}}$ is the equilibrium constant. By comparing with accurate simulations, this result is found to be exact for $d\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$, and we predict that it is exact for higher dimensions.