Optical melting experiments on oligoribonucleotides containing internal loops of three nucleotides (two nucleotides opposing one nucleotide) flanked by CG pairs show that the free energy of internal loop formation depends on the sequence of nucleotides within the loop. Internal loops with the potential for A.G and U.U mismatch pairing generally have more favorable free energies of formation than internal loops with potential AA, CC, AC, or UC pairings. In most cases, potential A.G and U.U pairings leaving a single unpaired nucleotide 5' to a flanking CG pair are most stable; those leaving a single nucleotide 3' to a flanking CG pair have intermediate stability. Internal loops of three nucleotides without potential A.G or U.U pairs are the least stable. The results provide an improved model for predicting the folding stabilities of internal loops of three nucleotides.