In the present study, a series of new double salt ionic liquids (DSILs) were successfully obtained All DSILs evaluated for herbicidal efficacy against common lambsquarters showed similar efficacy compared to parent ILs with a single anion (MCPA or dicamba). These results indicated that there were no noticeable synergistic effects for the analyzed systems comprising large amounts of dicamba anions. Interestingly, the system containing a smaller amount of dicamba (MCPA:dicamba at a molar ratio of 8:2) presented the highest biological activity of all the active substances tested and achieved 85 % fresh weight reduction compared with the control. It can be concluded that a small addition of dicamba can more than compensate for a reduction in the MCPA dose. Therefore, the total dose of pesticide can be effectively reduced to 25 % (from 400 to 305 g•ha -1 ) without any loss of efficacy. Most likely, the synergistic effects caused a noticeable increase in the toxicity of this system toward C. vulgaris compared with that of the parent ILs. This result confirms that a more thorough evaluation of the ecotoxicity of new compounds is crucial in the process of developing novel herbicidal formulations that are not only effective against target species but also safe for nontarget species.