Using this strategy we were able to scrutinize, for the first time, the electronic decoherence dynamics of the DNA base thymine in room temperature water. In this case, the decoherence occurs in ∼ 20 -30 fs with the early stages dictated by intramolecular vibrations, while the overall decoherence time due to interactions with solvent. Chemical substitution of the thymine with sugar and sugar phosphate reveals a recurrence in the coherence dynamics that is suppressed by hydrogen bonding between thymine and water. The vibrational modes most responsible for the decoherence are those that distort the thymine ring and carbonyl bonds. Increasing the temperature accelerates the contributions of solvent to the decoherence but leaves the early-time decoherence dynamics intact. For pure-dephasing, this early stage of the decoherence is accurately captured by the theory of decoherence time scales which does not require propagating the quantum dynamics.