A first-order decay of a diazonium salt, rate constant similar to 10(-3) s(-1), and a parallel, very fast, second-order coupling with the addition of pyrazolone, i.e., A --> S and A + B --> R, have been used to study macromixing in a stirred semibatch reactor. The experiments undertaken include two types of impeller, two Liquid heights, two feed positions, three feed rates, and three semitechnical scales of vessel from 0.3-0.6 m diameter. At high impeller speeds, all conditions give similar yields of around 90%. At lower speeds, typical of those found on the industrial scale, the yield fell as low as 65%. However, feeding into the impeller zone compared to the surface and using a larger 60 degrees pitch blade impeller rather than a standard Rushton turbine increased yields at similar values of power/volume to about 85%. Yields under geometrically similar conditions at each scale were the same when operating at-constant agitator speed. Such results fit in well with the concept that the same scale-up rule applies to macromixing as that for equal mixing times. It also matches the scale-up rule implied by the "network-of-zones" model developed by Mann and co-workers, though the absolute values of yield predicted differ widely from those obtained experimentally at low speeds and with surface addition. The results also show that improving bulk blending is a good strategy for increasing yields. The performance of a 60-ton industrial-scale reactor used to produce reactive dyes was significantly enhanced following this work by changing a feed point from the surface to the impeller region.