A device with femtoliter-scale chambers and controlled reaction initiation was developed for single-molecule enzymology. Initially separated substrate and enzyme streams were rapidly mixed in a microfluidic device and encapsulated in an array of individual microreactors, allowing for enzyme kinetics to be monitored with millisecond dead times and single-molecule sensitivity. Because the arrays of chambers were fabricated by micromolding in PDMS, the chambers were monodisperse in size, and the chamber volume could be systematically controlled. Microreactors could be purged and replenished with fresh reactants for consecutive rounds of observation. Repeated experiments with statistically identical initial conditions could be performed rapidly, with zero cross-talk between chambers in the array.