Differential scanning calorimetry was used to study the water of hydration in powder samples of deoxyadenosine (dA), a naturally occurring nucleoside. Though water of crystallization is present in samples which have not undergone heat treatment previously, dA was found to rehydrate at room temperature only at relative humidities (RHs) of 88% and higher. Rehydrated samples exhibited a single endothermic peak. At 95% RH, its activation energy was 1.61 ± 0.06 eV and had an enthalpy change of 190 ± 30 J/g. Experiments between 88 and 95% RH revealed that the energies are independent of RH, to within experimental error. This range of humidity corresponds to secondary hydration in DNA.