The absorption and optical rotatory dispersion of six dinucleoside phosphates and their corresponding nucleoside and nucleotide constituents were measured at pH 1, 7 and 11·5 at an ionic strength of 0·1. The optical measurements were made from 340 to approximately 210 m μ. The optical rotatory dispersion spectra of the dinucleoside phosphates were quite different from those of their constituents in both magnitude and shape. The effect of pH upon these spectra of the dinucleoside phosphates is large; whereas the effect on their absorption spectra is similar to that on their components. Using the hypochromism and the maximum difference in rotation between the dinucleoside phosphate and its constituents as criteria, one can classify the six dinucleoside phosphates as having either stacked bases or unstacked bases in the three solvents. It is found that neutral adenine, cytosine and guanine stack, whereas uracil does not. Charges on both bases cause unstacking, but a charge on one base need not. The dependence on temperature of the rotation of adenylyl (3′ → 5′) adenosine can be interpreted as the unstacking of the bases. The prediction of the properties of higher oligomers and polynucleotides from the properties of the 16 dinucleoside phosphates is discussed.