Proteins generally are only stable in vitro for short periods of time. This results in challenges during isolation and purification of recombinant proteins and reduces the shelf life of protein-based pharmaceuticals. Here we show that certain novel, biocompatible ionic liquids provide a stabilizing solvent for proteins, for example, cytochrome c, such that structure and activity are maintained even after 6 months of storage at room temperature. Normally, this protein would be rendered inactive after only 1 week in buffered aqueous solution. The effect of the ionic liquid solvent appears to be related to protection against hydrolysis. (c) 2006 Wiley Periodicals, Inc.