A new method is presented for attenuating time dependent composition or temperature variations in fluid streams. We refer to this new method as temporal mixing, or mixing in time. Temporal mixing involves passing fluid streams through a vessel or configuration of vessels that exhibits a certain residence time distribution (RTD). While conventional configurations, such as a buffer tank or stirred tank, provide some attenuation, new temporal mixing configurations are described that exhibit a higher degree of attenuation. A number of simple flow configurations exhibiting limited temporal mixing are analyzed, and a theoretical configuration exhibiting ideal temporal mixing is proposed. The ideal temporal mixing configuration is characterized by a perfectly flat RTD, which totally eliminates all periodic variations at the outlet of the configuration. Although this ideal case cannot be realized in practice, a vessel that closely approximates a flat RTD is described. Simulations based on real plant data are used to show that this vessel significantly improves the attenuation achieved over that attained with conventional methods. A number of temporal mixing vessels have been installed that demonstrate improved operation and a significant reduction in operating costs for world-class hydrogen facilities. (c) 2006 American Institute of Chemical Engineers.