This study employs a semi-continuous reactor system to investigate the solvent effect of gamma-valerolactone (GVL) on cellulose hydrolysis in hot-compressed GVL/water (HCGW) at 250 degrees C and 10 MPa. The primary liquid products from cellulose hydrolysis in HCGW at various GVL concentrations are collected and characterised. Both hydrolysis reaction rate and final glucose recovery are found to increase with GVL concentration, with a high glucose recovery of 91% achieved in the primary liquid products of cellulose hydrolysis in 10% GVL/water. Further analyses of the primary liquid products using high performance anion exchange chromatography with pulsed amperometric detection show the reductions in the isomers of glucose oligomers (with reducing end as fructose and mannose) and the increases in the glucose oligomers with low degrees of polymerisation, indicating that GVL addition enhances hydrolysis reactions but suppresses isomerization reactions in the solid phase. Similar effects are also found in the secondary reactions of the primary liquid products during cellulose hydrolysis at reduced flow rates. The experimental results from biomass hydrolysis clearly demonstrate that GVL/water co-solvent is an excellent reaction medium for biomass hydrolysis under acid-free conditions, achieving a near-complete sugar recovery (i.e., 93%) at a low GVL concentration of 5%.