We study the macromixing behavior of single and multi-orifice-impinging transverse (MOIT) jet mixers with crossflow, in particular, the overall mixing time and the back-splash mixing time of the injected flow with the crossflow, using the PLIF technique. It is found that for a given mixer configuration, there is a critical jet-to-crossflow velocity ratio r(c) at which the back-splash begins to occur. Further increase in the velocity ratio r leads to sharp increase in the back-splash mixing time, which can offset the intensification of the downstream mixing. The dimensionless overall mixing time decreases as r increases to reach either a plateau or a local minimum, and the corresponding r value represents the optimal velocity ratio ropt for the macromixing. The momentumratio of the two liquid streams is a key factor determining rc and ropt. For a larger scale mixer, a higher momentum ratio is required to achieve the optimal macromixing with the minimum dimensionless overall mixing time. (C) 2016 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.