The thermal properties of Poly(A+U) formed by an equimolar mixture of Poly A and Poly U in aqueous solutions of various concentrations of NaCl were studied calorimetrically. First, the measurements using the modified DSC were carried out and the transition enthalpy (ΔHm) was estimated for Poly(A+U) in the solutions of various salt concentrations. The heat of mixing of an equimolar mixture of Poly A and Poly U under the same experimental conditions as used in the DSC measurement was measured by using a LKB microcalorimeter at 298K; the results obtained were found to depend on salt concentration as well. The agreement between the enthalpy of formation of Poly(A+U) estimated from the microcalorimeter and the heat of transition determined by the DSC method was fairly close.