In the case of esfenvalerate/fenvalerate, the abundance in the commercial technical mixture is of 60% and 40%, for isomer I and II respectively. These percentages were similar to those found in thatch material and also in human breast milk samples, with 60% and 40% and 56% and 44% for isomer I and II in thatch material and human breast milk, respectively. In this case, no differential isomeric behavior was observed.