  NMR spectroscopy is an excellent tool for monitoring in-situ chemical
reactions. In particular, DOSY measurement is well suited to characterize
transient species by the determination of their sizes. However, here we bring
to light a difficulty in the DOSY experiments performed in out-of-equilibrium
systems. On such a system, the evolution of the concentration of species
interferes with the measurement process, and creates a bias on the diffusion
coefficient determination that may lead to erroneous interpretations.
  We show that a random permutation of the series of gradient strengths used
during the DOSY experiment allows to average out this bias. This approach, that
we name p-DOSY does not require changes in the the pulse sequences nor in the
processing software, and restores completely the full accuracy of the measure.
This technique is demonstrated on the monitoring of the anomerization reaction
of \alpha- to \beta-glucose.
