It is postulated that around a vacancy in diamond a reaction-volume, termed the catch-volume, exists such that an interstitial atom that enters this volume must recombine with the vacancy. It is further postulated that, owing to the relaxation of the atoms around the vacancy, the activation barrier E C that an interstitial atom has to scale when jumping into this volume can be different from the activation barrier E D for diffusion. A model is then developed to explain why the residual vacancies decrease with annealing temperature when annealing either electron-irradiated or ion-implanted diamonds. It is argued that this catch-volume is different, and can form in addition to the capture radii that are present around traps when elastic and/or Coulomb forces are present.