Application of the ab initio quantum mechanical calculations to assign matrix-isolation IR spectra of oxopyrimidines are discussed in the view of the recently published studies in this area. The examples presented in this review indicate that a considerable progress has been accomplished in the last few years due to the improvements of the experimental techniques as well as due to increased accuracy of the theoretical calculations. In discussing the examples we identify the discrepancies which still exist between the calculated and experimental frequencies and intensities. We also suggest how to improve the theoretical methodology to achieve better agreement with the experiment.