In this laboratory experiment, college students are introduced to dynamic combinatorial chemistry (DCC) and apply it to determine the substrate selectivity of acetylcholinesterase (AChE). Initially, the students construct a chemical library of dynamically interchanging thioesters and thiols. Then, AChE is added and allowed to select and hydrolyze the best substrate, thereby removing it from the equilibrium and subsequently amplifying it. Analysis of these processes is done by [superscript 1]H NMR analysis. The one-day bioorganic laboratory exercise introduces this new and rapidly growing concept in a realistic setting, using a target of biological significance. (Contains 5 figures and 6 notes.)