<p>Sorption of the most pharmaceutically significant methylxanthines, caffeine, theophylline and theobromine, on the metal-organic framework polymers (MOFs) MIL-121, MIL-53(Al), and MIL-88A with different metal centers (iron, aluminum) was studied in the present work. The MOFs were synthesized using known techniques. Their structures were confirmed using x-ray diffractometry and porosimetry. The content of the analyte sorbed on the MOFs was determined using UV-Vis spectrophotometry from the difference in the content of the compounds in the sample before and after treatment of the solution with the MOFs. Selective sorption of methylxanthines on these MOFs was demonstrated using the eluents MeCN, deionized water, EtOH (96%), and an MeCN:NH₄OAc buffer solution (5 mM, pH 4.5) in a 6:4 ratio. It was found that the sorption selectivity was due to the nature of the MOF metal center and the type of eluent. The difference in the sorption capacities indicated that solid-phase extraction of the xanthines and their selective separation were possible.</p>