The mechanism of the liquid phase glycerol hydrodeoxygenation-methanol reforming tandem reaction cycle over a Cu:Zn:Al catalyst is investigated. The hydrogen needed is provided in-situ via methanol reforming. In-situ DRIFTS of adsorbed methanol proved that methanol reforming proceeds via sequential methanol dehydrogenation to CO and further conversion to CO2 via water gas shift reaction. High hydrogen formation rate is achieved by increasing methanol to glycerol molar ratio. The elucidation of reaction pathways using mechanistic tests and in-situ IR spectroscopy showed that the first step of glycerol hydrodeoxygenation depends on hydrogen availability. Glycerol hydrodeoxygenation proceeds mainly via dehydration-hydrogenation route with hydroxyacetone as the main intermediate product. However at low methanol/glycerol molar ratio, the simultaneous formation of 1,2-propanediol via glyceraldehyde route cannot be excluded. 1,2-propanediol is stable product with low rate of further hydrodeoxygenation to 1-propanol. Though at low selectivities, ethylene glycol is the main degradation by product formed via direct hydrogenolysis of glycerol. (C) 2018 Elsevier Inc. All rights reserved.