We conclude that the hydrolysis of the internucleoside bond in these phosphodiesters involves two, parallel, reaction pathways: (a) a more or less concerted general base catalysed reaction; and (b) a two-step process, involving the rate determining general acid catalysed breakdown of the phosphorane monoanion intermediate 10. Near pH 7, in the presence of Im and ImH(+), the two pathways fun at comparable, but very slow rates. Ribonucleases logically combine the catalytic advantages of both pathways.