The formation of aluminium complexes of N‐acetyl, ‐chloroacetyl, ‐benzoyl and ‐o‐, ‐m‐ and ‐p‐nitrobenzoyl N‐phenylhydroxylamines has been studied potentiometrically in 1:1 aqueous acetone at constant ionic strength (μ = 0.5). Only the first two steps could be followed except in the case of N‐acetyl derivative. Values of K1 and K2 have been evaluated by projection‐strip method and those of K1, K2 and K3 by linear plots. The stabilities of the chelates vary linearly with ligand basicity and a discussion has been made to account for the observed facts. Copyright © 1971 Verlag GmbH & Co. KGaA, Weinheim