Disclosed are “acid-amide” calixarenes of formula (I) wherein: L is [—CH2—] or [—O—CH2—O—] and may be the same or different between each aryl group; R5 is H, halogen, or C1-C10 aliphatic hydrocarbyl group, C6-C20 aryl group, C6-C10 hydrocarbyaryl group, any of which may optionally be substituted by one or more halo or oxo groups or interrupted by one or more oxo groups, and R5 may be the same or different on each aryl group; R1 comprises an optionally protected carboxy group; two groups out of R2, R3, and R4 are H; the one group out of R2, R3, and R4 not being H comprises an amide group. The amide group may be linked to a second calixarene to form a dimer. Also disclosed are methods of use of such calixarenes for the purposes of metal sequestration, especially of lanthanides and actinides. Also disclosed are calixarene dimer derivatives of the calixarenes of the invention. Also disclosed are processes for preparing the calixarenes and dimers.