Chemical engines process mass flows to convert differences in chemical potential into work—the analog of heat engines processing heat flows to generate work from temperature differences. For isothermal endoreversible chemical engines constrained to finite‐time operation—examples of which include mass exchangers, electrochemical, photochemical, and solid‐state devices: (i) optimal cycle types for maximizing average power output, (ii) upper bounds on power production, and (iii) sensitivity of these results to mass‐transfer law, illustrated for linear and diffusive behavior, are derived.