The pseudobinary system LaAlxNi5-x has a composition limit at x approximate to 1.4. The standard molar enthalpies of formation Delta H-f(m)0(298.15 K) for this system were determined by solution calorimetry in HCl (c = 6 mol.dm(-3)) to be: -(129.4 +/- 6.5) kJ.mol(-1) for x = 0; -(159.5 +/- 15.9) kJ.mol(-1) for x = 0.25; -(185.9 +/- 6.7) kJ.mol(-1) for x = 0.375; -(191.8 +/- 12.4) kJ.mol(-1) for x = 0.5; and -(283.2 +/- 8.9) kJ.mol(-1) for x = 1. Uncertainties correspond to twice the standard deviation of the mean and include contributions from the scatter of the calorimetric results and electrical calibrations, as well as analytical uncertainties. Previous results for Delta H-f(m)0(LaNi5) fall into two groups; they are compared with the present determination. The plot of Delta H-f(m)0 against x is linear within experimental error. (C) 2000 Academic Press.