Abstract—Samples of composites with different porosity and surface roughness based on LaFe11.4Mn0.3Si1.3H1.6 (LFMSH) alloy powders were obtained, their magnetocaloric properties were studied by a direct method in cyclic magnetic fields μ0H = 1.2 T at a frequency of 2 Hz. The maximum value of the adiabatic temperature change in pure LFMSH powder was ΔT = 3 K at Т0 = 287 K in the sample cooling mode; for composite samples, this value turned out to be approximately 2 times lower than in the powder. The effect of Mn and H atoms on the electronic structure and local magnetic characteristics of the initial La(Fe,Si)13 alloy has been studied by the methods of the electron density functional theory. Replacing some of the Fe atoms with Mn reduces the total magnetic moment and slightly lowers the Curie temperature. Hydrogenation, on the contrary, leads to an increase in exchange interactions between Fe atoms located at the vertices of the icosahedron and an increase in the Curie temperature.