7-O-(4-Substituted benzyl)monensins (3a-g) were synthesized from monensin (1), and their lipophilicity, antibacterial activity, and Na⁺ ion permeability were examined. 7-O-(4-Ethylbenzyl)monensin (3a) showed the largest Na⁺ ion permeability, but 3c, f, g showed smaller Na⁺ ion permeability than 7-O-benzylmonensin (2) in spite of higher lipophilicity. An X-ray study of the sodium salt of 3e revealed that the benzyl group was located over the position between the D and E rings, and that the ethyl substituent on the benzyl group was close to the C(28) methyl group on the E ring.