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The successive addition of H atoms to CO in the solid phase has been hithertoregarded as the primary route to form methanol in dark molecular clouds.However, recent Monte Carlo simulations of interstellar ices alternativelysuggested the radical-molecule H-atom abstraction reaction CH3O + H2CO -> CH3OH+ HCO, in addition to CH3O + H -> CH3OH, as a very promising and possiblydominating (70 - 90 %) final step to form CH3OH in those environments. Here, wecompare the contributions of these two steps leading to methanol byexperimentally investigating hydrogenation reactions on H2CO and D2CO ices,which ensures comparable starting points between the two scenarios. Theexperiments are performed under ultrahigh vacuum conditions and astronomicallyrelevant temperatures, with H:H2CO (or D2CO) flux ratios of 10:1 and 30:1. Theradical-molecule route in the partially deuterated scenario, CHD2O + D2CO ->CHD2OD + DCO, is significantly hampered by the isotope effect in theD-abstraction process, and can thus be used as an artifice to probe theefficiency of this step. We observe a significantly smaller yield of D2CO + Hproducts in comparison to H2CO + H, implying that the CH3O-induced abstractionroute must play an important role in the formation of methanol in interstellarices. Reflection-Absorption InfraRed Spectroscopy (RAIRS) and TemperatureProgrammed Desorption-Quadrupole Mass Spectrometry (TPD-QMS) analyses are usedto quantify the species in the ice. Both analytical techniques indicateconstant contributions of ~80 % for the abstraction route in the 10 - 16 Kinterval, which agrees well with the Monte Carlo conclusions. Additional H2CO +D experiments confirm these conclusions.

First experimental confirmation of the CH3O + H2CO -> CH3OH + HCO reaction: expanding the CH3OH formation mechanism in interstellar ices