A previous investigation of the binary phase diagram (Cs, Rb)NO3 has confirmed the existence of two continuous solid solutions a(Cs1-x, Rbx)NO3 and b(Cs1-x, Rbx)NO3 stable in the temperature ranges T = (298 to 421) K and T = (439 to 503) K respectively. The mixing enthalpies of these solid phases, determined by a calorimetric method at T = (298.15 and 473.15) K, are presented for the first time. A positive and asymmetric deviation from ideality is observed. (C) 2014 Elsevier Ltd. All rights reserved.