Here we present a time-resolved crystallographic analysis of the hydrolysis of exo (Sp) guanosine 2',3'-cyclophosphorothioate by RNase T1. The use of a slow substrate and fast crystallization methods made it possible to perform the study with conventional data-collection techniques. The results support the idea that the hydrolysis reaction proceeds through a mechanism that is the inverse of the transesterification reaction. In addition, the structures provide an explanation for the differential behavior of RNase T1 towards exo- and endo-cyclic thiophosphates.