Novel K and Cs complexes with diphenyloxa-21-crown-7 (2,3,5,6-dibenzo-1,4,7,10,13,16,19-heptaoxacyclo-heneicosadiene-2,5; DPO21C7) are synthesized and isolated in a solid state, and their IR spectra (4000−400 cm−1) are studied. Only four ether O atoms of crown ether are involved in coordination. Conformation of a macrocycle in the complexes depends on the cation size. The potentiometric selectivity of diphenyloxa-21-crown-7 toward Cs+ ions was found to be the greatest.