The temperature dependence of the vapour pressures of solid alkali-metal fluoroborates MBF4 (M = Na, Rb or Cs) were experimentally determined using an improvised transpiration technique. The vapour pressure of NaBF4 could be represented by the following least-squares expressions: log(p/Pa) [NaBF4, orthorhombic] = 7.06(+/-0.03) - 3734(+/-360) (K)/T log(p/Pa) [NaBF4, monoclinic] = 6.03(+/-0.04) - 3192(+/-80) (K)/T The enthalpy pertaining to the transition from the orthorhombic to the monoclinic phase at 527 K, was found to be 10.4 kJ mol(-1). The measured vapour pressures of RbBF4 and CsBF4 could be represented by the following least-squares expressions: log(p/Pa) [RbBF4] = 6.21 (+/-0.04) - 4020(+/-87) (K)/T log(p/Pa)[CsBF4] = 7.77(+/-0.04) - 5209(+/-108) (K)/T By using the third law analyses of the data, Delta H-sub(o) of MBF4 (M = Na, Rb or Cs) at 298 K were determined to be 89.4+/-1.5, 123.4+/-1.9 and 135.7+/-0.8 kJ mol(-1), respectively. (C) 2006 Elsevier B.V. All rights reserved.