NOVELTY - Pretreating lignocellulosic biomass containing alkali and/or alkaline earth metal (AAEM), comprises: providing lignocellulosic biomass containing AAEM; determining the amount of AAEM present in the lignocellulosic biomass; identifying, based on the determining, the amount of a mineral acid sufficient to completely convert the AAEM in lignocellulosic biomass to thermally-stable, catalytically-inert salts; and treating the lignocellulosic biomass with the identified amount of the mineral acid, where the treated lignocellulosic biomass contains thermally-stable, catalytically inert AAEM salts.    USE - The method is useful for pretreating lignocellulosic biomass containing AAEM to obtain bio-oil product (claimed).    ADVANTAGE - The method provides the bio-oil product having: sugar yield of 2-9 fold compared to a bio-oil product produced from the provided lignocellulosic biomass which is not subjected to the treating, or sugar yield of 18-25 wt.% compared to a bio-oil product produced from the provided lignocellulosic biomass which is not subjected to the treating having a sugar yield of only 2-6 wt.%; and light oxygenated compound yield which is decreased by 8-15 wt.% compared to a bio-oil product produced from the provided lignocellulosic biomass which is not subjected to the treating. The pretreatment process yields thermally-stable salts that significantly reduce the catalytic activity of the AAEM, where the salts formed by reacting AAEM cations with mineral acids not only passivate AAEMs that normally catalyze fragmentation of pyranose rings, but also form weak acids that buffer the system at pH levels that favor glycosidic bond breakage. The method utilizes chemical treatment that converts the AAEM into a form that is no longer catalytically active. The chemical treatment is feasible and cost-effective and employs very modest amounts of acid that is equal to the molar quantity of AAEM in the biomass, and moreover, there is no waste acid disposal, because the acid is completely converted into thermally stable salts.