The studies on the oscillatory structure of adsorption spectra of pyrimidine bases of nucleic acids isolated in Ar matrix at 11 K are described. To clear up the importance of molecule isolation in the matrix, amorphous films of the materials studied were investigated experimentally at 11 and 77 K. The work was carried out using the low temperature optic attachment developed by the authors. The long wavelength band of cytosine and deuterocytosine in the matrix is shown to consist of two bands: 1) lambda max = 267 nm with oscillatory progressions of 500 and 400 cm-1, respectively, and 2) lambda max = 280 nm with progression approximately 800 cm-1. The first pi pi*-absorption band of 1-methylcytosine has a single oscillatory progression 470 cm-1. Thymine and uracil in Ar matrices form diffuse structural spectra of 630 and 660 cm-1, respectively. The oscillatory progressions are attributed to the oscillatory frequencies of the pyrimidine molecule ring oscillations in the excited state. The annealing of the matrix results for all the materials in smearing the oscillatory molecule structure up to its complete vanishing. In film samples the oscillatory structure is not seen at low temperatures.