AbstractA series of 14C‐labelled benzoic acids, chosen to permit assessment of the role of pKa and lipophilicity in determining movement in plants of these herbicide analogues, was synthesised and their phloem translocation investigated. Following application of substituted benzoic acids to castor bean, Ricinus communis L., by injection into the petioles, the compounds of intermediate lipophilicity (2‐fluoro‐, 4‐chloro‐ and 3,4‐dichlorobenzoic acids) gave highest concentrations in phloem exudates; 4‐methyl‐2,3,5,6‐tetrafluoro‐ and pentafluorobenzoic acids were less well translocated, perhaps because their pKa values are much less than those of the other benzoic acids studied. The polar 4‐ureidobenzoic acid and the lipophilic 3‐(4‐methylphenoxy)benzoic acid were much less efficiently translocated in phloem. These results are similar to those previously obtained for phenoxyacetic acids, and provide further support for the role of ionisation in the accumulation and retention of chemicals in phloem sieve tubes.