The penetration of the wax barrier of plant cuticles is the rate-limiting step for the uptake of surface applied pesticides. Optimizing the uptake rates of active ingredients across this barrier is a significant objective of the rational design of pesticide formulations. We used an ATR-FTIR-based method for determining the diffusion coefficients derived from Fickian diffusion kinetics in paraffin wax of model diffusants differing in chemical structure, lipophilicity, and size at different temperatures. At 25 °C, the diffusion coefficients of 4-cyanophenol and heptyl parabene were 1.48 × 10–15 and 1.73 × 10–15 m2 s–1, respectively, and increased from 25 to 40 °C yielding activation energies of 54 kJ mol–1 and 71 kJ mol–1, respectively. In contrast, much higher diffusion coefficients were obtained for the accelerating adjuvants methyl oleate (1.49 × 10–13 m2 s–1) and C12E2 (4.52 × 10–14 m2 s–1), exhibiting entirely Fickian diffusion kinetics. The diffusion coefficients in wax are 3–5 orders of magnitude lower than diffusion coefficients in water predicted for these molecules, which illustrates the massive barrier the cuticle forms against the uptake of solutes. The current work demonstrates ATR-FTIR’s suitability as a new approach to optimizing pesticide formulations by quantitatively investigating active ingredient and adjuvant mobility in thin wax films.