  Deuterium fractionation in molecular ions, in particular HCO+, has been
extensively used to estimate the degree of ionization in molecular clouds. This
paper reviews recent work on ionization degree in homogeneous clouds. We will
show that the N(DCO+)/N(HCO+) column density ratio furnishes a measurement of
x(e) only in regions where CO is not significantly depleted, thus in the outer
skirts of dense cloud cores. To probe x(e) deep inside the clouds, one has to
gauge deuterium enhancement in molecular ions with parent species not affected
by depletion (e.g. N2H+), and rely on chemical models which take into account
the cloud density structure. Unlike N(DCO+)/N(HCO+), the N(N2D+)/N(N2H+) column
density ratio is predicted to considerably increase with core evolution (and/or
the amount of CO depletion), reaching large values (> 0.2) in cloud cores on
the verge of forming a star.
