New crown ether dyes carrying two pendent anionic side-arms were synthesized for the extraction-spectrophotometry of alkaline earth metal ions. In the extraction of alkaline earth metal ions by these dianionic reagents, size recognition by the crown ether ring was more remarkable than m the case of alkali metal ion extraction by a similar type of monoanionic reagents. Dramatic changes in metal selectivity were observed when the nature of the anionic side-arm was changed while the crown ether skeleton was kept the same. The structure/selectivity relationship is discussed in terms of "chelate" and "intramolecular ion-pair" formation. Typically, when the basicity of the pendent anions was relatively high and a six-membered chelate was structurally possible for the pendent anions and the crown-bound metal, the extraction of calcium was favored by up to a factor of 3000 in the ratio of the Ca/Ba extraction constants for reagents of the diaza-18crown-6 type. In contrast, the reagents which had pendent anions with only poor coordination ability for metal ions seemed to form complexes of the ion-pair type, and calcium ion was lo5 times less extractable than barium ion for the same diaza-18-crown-6 skeleton. Strontium ion seemed to be extracted most effectively when the extracted complex assumed properties intermediate between the chelate and intramolecular ion-pair.