  Deuterium enrichment of interstellar methanol is reproduced experimentally
for the first time via grain-surface H-D substitution in solid methanol at an
atomic D/H ratio of 0.1. Although previous gas-grain models successfully
reproduce the deuterium enrichments observed in interstellar methanol molecules
(D/H of up to 0.4, compared to the cosmic ratio of $\sim 10^{-5})$, the models
exclusively focus on deuterium fractionation resulting from the successive
addition of atomic hydrogen/deuterium on CO. The mechanism proposed here
represents a key route for deuterium enrichment that reproduces the high
observed abundances of deuterated methanol, including multiple deuterations.
