There is growing interest in the use of natural products such as caffeine and theobromine for control of a variety of pest species. We hypothesized that the limited water solubility of these compounds limits their effectiveness. To overcome this hurdle, we evaluated the use of sodium benzoate as an adjuvant to increase the solubility and potency of methylxanthines. Our results indicated that sodium benzoate increased the toxicity of a methylxanthine mixture to rodents but not to canids. These results indicate that sodium benzoate has potential for increasing the efficacy and selectivity of methylxanthine-based rodenticides. As methylxanthines and sodium benzoate are plant-derived natural products, potential applications for methylxanthine and sodium benzoate based pesticides include organic food production.