In an effort to identify novel ligands possessing high affinity and selectivity for the A2B AR subtype, we further investigated the class of 3-aryl[1,2,4]triazino[4,3-a]benzimidazol-4(10H)-ones V, previously disclosed by us as selective A1 AR antagonists. Preliminary assays on a number of triazinobenzimidazoles derived from our "in-house" collection revealed that all the derivatives selected showed significant affinity at A 2B AR, no affinity at A3 AR, and various degrees of selectivity toward A1 and A2A ARs. Investigation of a new series featuring modified substituents at the 10-position (4′-chlorophenyl or phenylethyl groups), and a chlorine atom at the 7-position (X) of the triazinobenzimidazole nucleus, yielded highly potent and selective A 2B AR antagonists. The presence of a pendant 3-phenyl ring appears to hamper the interaction with A2A AR, conferring high A 2B/A2A AR selectivity. Derivative 13 (X = Cl, R = C 6H5) is the most potent and selective compound, with an IC50 of 3.10 nM at A2B AR and no affinity at A 1, A2A, and A3 ARs