The magnetic circular dichroism and circular dichroism spectra of cytidine, isocytidine, 2,5'-O-cyclo-2',3'-O-isopropylidine uridine, and 2,2'-O-cyclouridine were measured in the wavenumber region of 30000 -50000 cm-1. On the basis of the experimental results, the tautomerism of cytidine and isocytidine was discussed. The transition energies, the oscillator strengths, and the Faraday parameters were calculated within the framework of the INDO approximation. The calculated results are in good agreement with the experimental results. And then the spectral assignment of the pyrimidin nucleosides, together with those of cytosine and isocytosine, were discussed on the basis of the experimental and theoretical results.