The results of a detailed calorimetric study of the NaNO3/KNO3 phase behaviour as a function of cation composition are presented. We conclude that the lambda-transition in NaNO3 is significantly depressed and broadened by the presence of K+ ions. The alpha-beta-transition in KNO3 is similarly affected. The possibility is considered that the depression of the lambda-transition by addition of solute K+ ions is related to random field effects on model lattice transitions involving a continuous order parameter. Some brief remarks are made to contrast the fast ordering kinetics of the lambda-transition with the slow kinetics, and structural arrests, obvious in supercooling liquids. (C) 2008 Elsevier B.V. All rights reserved.