Interaction in the systems LaNi3Cu2-H-2 and LaNi2Cu3-H-2 was studied by Cal
vet calorimetry. The effect of hydrogen and of thermal treatment in a hydro
gen atmosphere on the structure of the LaNi5-xCux metallic matrix, where x
= 2, 3, was studied. It was shown that under isothermal conditions can coex
ist at least two forms of the same compound with different properties.