AbstractThe phloem translocation of an homologous series of ω‐(1‐[1‐4C]naphthoxy)alkanoic acids relative to that of applied [6,6′‐3H] sucrose was measured inRicinus communisvar. Gibsonii. Relative phloem mobility decreased with increase in octan‐l‐ol/water partition coefficients. However, non‐ionisable compounds with partition coefficients similar to the readily translocated acids were not phloem mobile. These results are explained in terms of an ion‐trap mechanism subject to limitations on the partition coefficients of the acids and pos