The use of certain ionic liquids (ILs) as pretreatment solvents for lignocellulosic biomass has gained great interest in recent years due to the IL's capacity for efficient cellulose dissolution in aqueous solution as compared to other common pretreatment techniques. A fundamental understanding on how these ILs in aqueous environments act on cellulose, particularly at lower IL concentrations with water as a cosolvent, is essential for optimizing pretreatment efficiency, lowering pretreatment cost, and improving IL recyclability. The IL 1-ethyl-3-methylimidazolium acetate ([C(2)C(1)Im][OAc]) is one of the most efficient cellulose solvents known, greatly altering cellulose structure for improved enzymatic saccharification. To understand the role of water as a cosolvent with [C(2)C(1)Im][OAc], we investigated the dissolution mechanism of microcrystalline cellulose, type I-beta in different [C(2)C(1)Im][OAc]:water ratios at room (300 K) and pretreatment (433 K) temperatures using all atom molecular dynamics (MD) simulations. These simulations show that 80:20 ratios of [C2C1Im] [OAc] :water should be considered as "the tipping point" above which [C(2)C(1)Im][OAc]:water mixtures are equally effective on decrystallization of cellulose by disrupting the interchain hydrogen bonding interactions. Simulations also reveal that the resulting decrystallized cellulose from 100% [C(2)C(1)Im][OAc] begins to repack in the presence of water but into a less crystalline, or more amorphous, form.