By examining the first step of group II intron splicing in the absence of the second step, we have found that there is an interplay of three distinct reactions at the 5'-splice site: branching, reverse branching, and hydrolytic cleavage. This approach has yielded the first kinetic parameters describing eukaryotic branching and establishes that group II intron catalysis can proceed on a rapid timescale. The efficient reversibility of the first step is due to increased conformational organization in the branched intermediate and it has several important mechanistic implications. Reversibility in the first step requires that the second step of splicing serve as a kinetic trap, thus driving splicing to completion and coordinating the first and second step of splicing. Facile reverse branching also provides the intron with a proofreading mechanism to control the fidelity of 5'-splice site selection and it provides a kinetic basis for the apparent mobility of group II introns.