The potential of selected surfactants and other adjuvants to increase cuticular penetration of foliar applied active ingredients was investigated under defined environmental conditions (RH, T). The herbicide cyanazine and the weak acid NAA were selected as model compounds. The time course of cyanazine penetration through apple leaf cuticles in the presence of different adjuvants varied from no penetration within 5 days to almost quantitative penetration within 5 hours. Effective adjuvants are distinguished as donor adjuvants, that produce a liquid film on the surface, and accelerator adjuvants, that additionally can penetrate into and increase solute mobility in the cuticle. Adjuvant efficacy in both groups differs significantly among products and is characterized according to chemistry and concentration, technical product quality, environmental factors, and plant organ (leaf/fruit) and species, respectively. An example is given to combine adjuvants to manipulate the time course of penetration of NAA. The limited use of HLB as a general indicator for the efficacy of adjuvants at the cuticular level is discussed.