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Solution plasma or in-liquid plasma, which is generated by gas-phase discharge within bubbles in a solution, is an exciting reaction field for biomass conversion. However, it is not fully elucidated how the solution plasma works to degrade biomass or how biomass is degraded in it. In this study, various saccharides and alcohols, mainly sucrose, were treated in solution plasma using a high-voltage pulse power supply to study the degradation mechanisms. Hydrolysis and gasification were observed in the solution-plasma treatment of sucrose. The former was mainly influenced by the water temperature, and the latter was mainly influenced by the discharge power. Therefore, it was inferred that hydrolysis occurred in the hot-compressed water region around the plasma, and gasification occurred at the interface between the plasma and water. Gasification of saccharides and alcohols produced H2-rich gases, but gasification was faster for high-volatility alcohols and slower for non-volatile saccharides. The formation of H2-rich gas can be attributed to H2 formation by the water–gas shift reaction of CO and direct H2 formation from water, in addition to H2 from the sample. [ABSTRACT FROM AUTHOR]