The Tetrahymena group I ribozyme's oligonucleotide substrate, CCCUCUA(5), forms six base pairs with the ribozyme's internal guide sequence (IGS, 5'GGAGGG) to give the P1 duplex, and this duplex then docks into the active site via tertiary interactions. Shortening the substrate by three residues to give UCUA(5) reduces the equilibrium constant for P1 docking by approximately 200-fold even though UCUA(5) retains all the functional groups known to be involved in tertiary interactions [Narlikar, G. J., Bartley, L. E., Khosla, M., and Herschlag, D. (1999) Biochemistry 38, 14192-14204]. Here we show that the P1 duplex formed with UCUA(5) engages in all of the major tertiary interactions made by the standard P1 duplex. This suggests that the destabilization is not due to disruption of specific tertiary interactions. It therefore appears that the weaker docking of UCUA(5) arises from the increased conformational freedom of the undocked P1 duplex, which has three unpaired IGS residues and thus a larger entropic cost for docking. Further, a 2'-methoxy substitution at an IGS residue that is base-paired in the standard P1 duplex with CCCUCUA(5) but unpaired in the P1 duplex with UCUA(5) destabilizes docking of the standard P1 duplex approximately 300-fold more than it destabilizes docking of the P1 duplex formed with UCUA(5). These results suggest that fixation of groups in the context of a rigid duplex may be a general strategy used by RNA to substantially increase interaction specificity, both by aiding binding of the desired functional groups and by increasing the energetic cost of forming alternative interactions.