AbstractThe phytotoxicity and wetting ability of 61 surface‐active agents of known chemical structure have been tested on the leaves of two varieties of apple and two of plum.Surface‐active materials containing a branched alkyl chain in the hydrophobic portion of their molecule are better wetters than corresponding materials containing a straight alkyl chain. In any homologous series of materials, wetting ability increases with increase in size of the alkyl group. With the non‐ionic ethylene oxide condensates, maximum wetting occurs with those materials containing the minimum number of ethylene oxide groups conferring water solubility.Phytotoxicity appears to be governed by the nature of the ionic charge, by the physical size of the molecules or ions and to a lesser extent by the nature of the gegenion. All non‐ionic materials cause little damage on apple and plum leaves, anionic and cationic materials cause variable damage depending on their chemical structure and on the nature of the leaf surface. In any homologous series of surface‐active agents the phytotoxicity appears to pass through a maximum as the molecular size is progressively increased. Since this maximum usually occurs at fairly short alkyl chain lengths (dodecyl or below), for most practical purposes phytotoxicity decreases as the size of the alkyl group increases.These results are discussed in the light of the conclusions reached in Part II of this series, and the physical and chemical factors governing phytotoxicity are considered.