UV irradiation ( λ = 254 nm) of thymine and uracil in aqueous solution containing salts of phosphinic acid (hypophosphites) results in the formation of the corresponding dihydropyrimidine derivatives. The peculiarities of this new photochemical reaction consist of a specificity towards 2,4-dioxopyrimidines, a high quantum yield and a neutral pH optimum. The quantum yield of photoconversion of thymine at pH 7.0 is equal to 1.9 × 10 −2 in 1 M NaH 2PO 2; it is diminished to 8.5 × 10 −3 in 0.1 M NaH 2PO 2. The same decrease in quantum yield with concentration is also found for uracil and uridine; quantum yields of their transformations in 0.1 M NaH 2PO 2 at pH 7.0 are 6.6 × 10 −2 and 1.5 × 10 −1 respectively. The mild conditions and high quantum yields characteristic of the photoinduced reaction above open up the possibility of obtaining DNA molecules which contain among pyrimidine photoproducts mainly dihydropyrimidine residues. A correlation between various types of thymine modifications in UV-irradiated double-stranded DNA (dimers and dihydrothymidine residues) and the amplitude of the intense negative band in the circular dichroism (CD) spectra specific for DNA liquid crystalline dispersions has been established. A possible application of this base-specific modification to the investigation of nucleic acids is considered.