This chapter discusses the stabilization of Trichoderma ressie cellulase (Tri-Cel) in ionic liquids (ILs) to enable the in situ hydrolysis of cellulosic and lignocellulosic substances. It is well recognized that enzymes tend to lose their activities in ILs, but several methods have been used to increase or minimize the loss of activity in ILs. In this study, cellulase was therefore tested in several ILs. This approach opens an insight for further studies to discover more about the effects of ILs on cellulase and their interactions in the aqueous system. It can also offer successful manufacturing and processing of different biomass biofuels. © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2021.