Aldose reductase (AR) is a member of the aldo-keto reductase superfamily. Due to its ability to catalyze the formation of sorbitol from glucose during hyperglycemic and hypertonic stress, the aldose-reducing property of AR has been accepted as its main physiological and pathological function. Nonetheless, AR is a poor catalyst for glucose reduction and displays active-site properties unexpected of a carbohydrate-binding protein. We, therefore, examined the catalytic properties of AR with a series of naturally occurring aldehydes, compatible in their hydrophobicity to the large apolar active site of the enzyme. Our results show that recombinant human AR is an efficient catalyst for the reduction of medium- to long-chain unbranched saturated and unsaturated aldehydes. The enzyme displayed selective preference for saturated aldehydes, such as hexanal, and unsaturated aldehydes, such as trans-2-octenal and nonenal as well as their 4-hydroxy derivatives. Short-chain aldehydes such as propanal and acrolein were reduced less efficiently. Branched derivatives of acrolein or its glutathione conjugate (GS-propanal) were, however, reduced with high efficiency. In the absence of NADPH, the alpha, beta unsaturated aldehydes caused covalent modification of the enzyme. On the basis of electrospray mass spectrometric analysis of the wild-type and site-directed mutants of AR (in which the solvent exposed cysteines were individually replaced with serine), the site of modification was identified to be the active-site residue, Cys 298. The unsaturated aldehydes, however, did not modify the enzyme bound to NADPH and did not inactivate the enzyme during catalysis. Modeling studies indicate that the large hydrophobic active site of AR can accommodate a large number of aldehydes without changes in the structure of the binding site or movement of side chains. High hydrophobicity due to long alkyl chains or apolar substituents appears to stabilize the interaction of the aldehyde substrates with the enzyme. Apparently, such hydrophobic interactions provide substrate selectivity and catalytic efficiency of the order achievable by hydrogen bonding. Since several of the aldehydes reduced by AR are either environmental and pharmacological pollutants or products of lipid peroxidation, the present studies provide the basis of future investigations on the role of AR in regulating aldehyde metabolism particularly under pathological states associated with oxidative stress and/or aldehyde toxicity.