A mechano-chemical method was used to synthesize solid solution Na(BH4)(1-x)Br-x with 0 <= x <= 1. Samples with compositions of x <= 0.333 were annealed, in order to form a single phase material. Bromide substitution leads to smaller unit cell size and lower temperature and enthalpy of the order-disorder phase transition of NaBH4. There is a linear relation between the amount of substitution, the temperature, the enthalpy and the kinetics of the phase transition. This linear relation between enthalpy and amount of substitution can be expressed by the function Delta H = -6.268x + 1.206 where xis the amount of substitution and Delta H is the enthalpy. Crown Copyright (C) 2017 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. All rights reserved.