Spray adjuvants are products used with pesticides to improve their application or effectiveness. While exempt from federal pesticide registration by the US Environmental Protection Agency (US EPA), California law requires their registration as pesticides before sale or use. The California Department of Pesticide Regulation evaluated the suitability and acceptability of the United Nations Globally Harmonized System of Classification and Labelling of Chemicals (GHS) Mixtures Equation for predicting the acute oral toxicity of spray adjuvants as an alternative to requiring in vivo animal toxicity testing. The GHS Mixtures Equation estimates the acute oral toxicity (LD₅₀) of a mixture by combining individual LD₅₀ values of component ingredients. This study compared the results of the acute oral toxicity categories derived from the GHS Mixtures Equation with toxicity categories determined from in vivo oral studies for 51 mixtures. The results showed that the availability of LD₅₀ values for the relevant ingredients in the mixture limited the equation’s predictivity. The equation predicted the LD₅₀ values for 67% (N = 34) of the dataset. When these datasets were further compared against the US EPA toxicity classification system, the GHS Mixtures Equation results achieved 71% concordance, with toxicity either over-classified (6%) or under-classified (22%) by one category. Most discordant results were for spray adjuvant mixtures with LD₅₀ > 2000 mg/kg (limit test), particularly when 20% or more of the components had unknown LD₅₀s (N = 5). Finally, when compared against the US Occupational Safety and Health Administration’s GHS classification system, our results showed 82% concordance with 15% over-classified and 3% under-classified by one toxicity category (N = 34). Overall, the findings indicate that the GHS Mixtures Equation can reliably predict acute oral toxicity in low-toxicity mixtures, provided that all active ingredients have known LD₅₀s and that fewer than 20% of the components have unknown LD₅₀s.