The analysis of the IR v(s)(PO32-) band shape of disodium deoxycytidine 5'-monophosphate, 5'-dCMP, in (H2O)-H-2 and H2O solutions at different concentrations, 0.005-0.90 mol dm(-3), and temperatures, 7-70 degreesC, provides information on the relaxation and dynamics of the phosphate group. Self-association of this mononucleotide is detected at concentrations higher than similar to0.28 mol dm(-3). The relaxation, dominantly vibrational, is similar to0.31 ps faster in H2O than in (H2O)-H-2 and similar to70 fs faster than for the mononucleotide 5'-CMP. Kubo-Rothschild's and Oxtoby's models for vibrational dephasing describe the IR time-correlation functions up to similar to3 ps. The stretched exponential model takes values for the dispersion parameter, alpha, close to 1, i.e., approaches the Markovian Kubo-Rothschild's model, and random fluctuations prevail at concentrations less than or equal to0.28 mol dm(-3). The modulation parameters resulting from these models indicate that the v(s)(PO32-) oscillator is under a fast modulation regime at low concentration but is slowing to an intermediate modulation regime as concentration approaches similar to0.3 mol dm(-3).