Thioredoxin reductase (TrxR) is the enzyme responsible for the reduction of thioredoxin (Trx), a ubiquitous redox mediator in cells.  Three forms of TrxR are found in nature, including a lightweight class commonly found in prokaryotes and lower eukaryotes.  This form of TrxR is a flavoprotein that also contains a redox-active disulfide.  It has been shown conclusively that lightweight TrxR catalysis is supported by a conformational change, which rotates the NADPH-disulfide domain with respect to the FAD containing domain.  In this report, protein film voltammetry (PFV) was used to characterize the redox chemistry of three bacterial lightweight TrxRs from Escherichia coli, Thermoplasma acidophilum and Archaealgulbidis fulgidus in the absence of substrate.  The resulting voltammograms display broad electrochemical signals that sharpen upon gentle heating.  The data are interpreted in terms of an electrochemical model relating voltammetric signal shape to a distinct conformation of lightweight TrxRs.