Heterotrimeric arylboroxines can be favorably formed by designing one of the arylboronic acid monomers to contain a pendant Lewis base. Using density functional theory (B3LYP//6-311+G*) calculations including Poisson-Boltzmann implicit solvent, we found that AB, trimeric arylboroxines were thermodynamically favored over A(2)B, A(3), or B-3, where A and B are monomeric arylboronic acids with and without a pendant Lewis base, respectively. The most stable AB, trimers were formed when the B monomer contained electron-withdrawn substituents, particularly halogens in the para-position or pi-acceptors in the meta-position. On the other hand, adding different para-substituents to the A monomer did not significantly change the energetics. Our calculations also suggest that ABC trimers with three different monomers will not be significantly favored over AB, trimers when making small electronic perturbations, by changing the substituents on each monomer.