In order to obtain information about the thermodynamic interactions for the poly(A) poly(U) duplex (duplex-I)-ethidium bromide (EB) system in dilute solution, the heats of mixing, Delta(mix)H(1), and UV spectra for the duplex-I-EB system were measured. From UV spectral measurements, the binding constant, K-1, and the number of duplex-I base pair occluded by EB, n(1), were estimated to be about 2.0 x 10(5) dm(3) mol(-1) and 2.5, respectively. The change in net enthalpy, Delta H-1’, obtained using Delta(mix)H(1), K-1, and n(1) values was estimated to be about -28 kJ (mel of EB)(-1). Using Delta H-1’ and K-1 values, the changes in free energy, Delta G(1)(0), and entropy, Delta S-1, based on an interaction of EB with duplex-I were estimated from Delta G(1)(0) = -RT In K-1 and Delta S-1 = (Delta H-1’ - Delta G(1)(0))/T, respectively.