  Extremely high deuteration of several molecules have been observed around low
mass protostars since a decade. Among them, formaldehyde and methanol present
particularly high deuteration, with observations of abundant doubly and triply
deuterated forms. Both species are thought to be mainly formed on interstellar
grains during the low temperature and dense pre-collapse phase by H and D atom
additions on the iced CO. We present here a theoretical study of the
formaldehyde and methanol deuteration obtained with our gas-grain model,
GRAINOBLE. This model takes into account the multilayer nature of the mantle
and explores the robustness of the results against the uncertainties of poorly
constrained chemical and surface model parameters. The comparison of the model
predictions with the observations leads to two major results: i) the observed
high deuteration is obtained during the last phase of the pre-collapse stage,
when the density reaches 5 10^6 cm^-3, and this phase is fast, lasting only
several thousands years. ii) D and H abstraction and substitution reactions are
crucial in making up the observed deuteration ratios; This work shows the power
of chemical composition as a tool to reconstruct the past history of
protostars.
