The enthalpies of solution of (NH4)(2)B4O7.4H(2)O (cr) and NH4B5O8.4H(2)O (cr) in approximately 1 mol dm(-3) aqueous hydrochloric acid solution, as well as the enthalpy of solution of NH4Cl (cr) in aqueous (hydrochloric acid + boric acid) have been measured by using a microcalorimeter. From these results and other auxiliary data, the standard molar enthalpies of formation Delta(f)H(m)(0) of (NH4)(2)B4O7.4H(2)O (cr) and NH4B5O8.4H(2)O (cr) have been derived to be (-4313.74 +/- 3.26) and (-4703.56 +/- 4.03) kJ mol(-1) at 298.15 K, respectively. Some other thermodynamic properties of the title borates, such as the standard molar entropies of formation and the standard molar entropies have also been calculated. (C) 2004 Published by Elsevier B.V.