The molecular chaperone Hsp33 undergoes unfolding and activation in response to oxidative stress conditions in the cytoplasm. These changes primarily involve Hsp33's C-terminal redox switch domain, which contains a flexible linker and a redox-sensitive center. In its unfolded state, Hsp33 interacts with client proteins, preventing them from misfolding and aggregating. Upon restoration of the normal, reducing environment in the cytoplasm, Hsp33 refolds and releases its clients to the DnaK chaperone system. Now Jakob and colleagues reveal how Hsp33 exerts its protective effect on protein substrates. The authors use peptide arrays with sequences from known client proteins to determine binding specificity. Together with solution studies using peptides of known structures, they find that Hsp33 prefers peptides with secondary structure over unfolded ones. This suggests that Hsp33 interacts with early unfolding intermediates and explains how it avoids binding its own unfolded redox switch domain. By limited proteolysis or hydrogen-deuterium exchange, coupled to MS analyses, the authors establish that it is the linker region within the unfolded redox switch domain that binds substrates, and notably, this interaction leads to stabilization of the linker itself. Finally, the authors follow a model substrate protein: upon refolding of Hsp33, the interacting regions within the substrate become destabilized, indicating that they undergo further unfolding, to be refolded by DnaK. Altogether, these results provide comprehensive insight into how this ATP-independent chaperone uses reversible order-to-disorder transitions to protect substrates from misfolding and to foster productive refolding. (Cell doi:10.1016/j.cell.2012.01.045, published online 2 March 2012)