La(Fe,Si) 13 alloys display a giant magnetocaloric effect when a magnetic field is applied near the Curie temperature T C . However, to use these alloys for domestic refrigeration based on magnetic cooling, it is vital to increase T C near to the room-temperature range while simultaneously maintaining a large magnetocaloric effect. With this aim, we studied the effect of interstitial carbon on the microstructure and magnetocaloric effect in LaFe 11.6 Si 1.4 C x (x ¼ 0-0.4). The investigation was carried out in cast samples annealed for seven days at 1323 K. The study of microstructure shows that annealing led to about 90 wt. % of 1:13 magnetocaloric phase. Magnetization data revealed that the addition of carbon leads to an increase in T C and a decrease of the thermal hysteresis width. For x > 0.2, the magnetic transition changes from first-order to second-order, with a corresponding reduction in magnetocaloric effect. A small amount of C (x up to 0.2) improves the magnetocaloric properties of the parent alloy La(Fe,Si) 13 , and, furthermore, the carbon addition leads to an increase in the thermal stability of hydrided LaFe 11.6 Si 1.4 C x . The onset of hydrogen desorption increases from 460 K for the x ¼ 0 (carbon-free alloy) to 500 K and 540 K, respectively, forMagnetic refrigeration, based on the magnetocaloric effect (MCE), is a competing technology to conventional gas-compression refrigeration at room temperature, because it offers improved efficiency and reduced environmental impact. 1,2 A promising magnetic refrigerant with a giant MCE is the intermetallic LaFe 13-x Si x , which crystallizes in the cubic NaZn 13 -type structure (the so-called 1:13 phase) for x varying from 1.2 to 2.5. 3-6 The LaFe 11.6 Si 1.4 compound has a sharp first-order itinerant electron metamagnetic (IEM) transition with Curie temperature T C ¼ 195 K. At temperatures just above T C , the IEM transition can be induced by applying a modest magnetic field. 4 Increasing x has the benefit of shifting T C up to 240 K, but this changes the transition from first-order to second-order and greatly reduces the MCE. T C can also be raised to room temperature and above by substituting cobalt for iron 7,8 or by the interstitial insertion of hydrogen (H), carbon (C), or boron (B) atoms. 9-13 In case of H insertion, the first-order nature of the IEM transition can be maintained at room temperature and, hence, the giant MCE; 14,15 however, H is very mobile and desorbs at about 450 K, 16 which is not far from room temperature, and, depending on the application, this could be a drawback.In this context, very little attention has been paid so far to improve the thermal stability of LaFe 13-x Si x H y against H desorption. Motivated by this, we report how the addition of C greatly improves stability of LaFe 13-x Si x H y to overcome this problem. First, we focus on the effect of C addition in LaFe 11.6 Si 1.4 (H-free) regarding magnetocaloric properties in order to establish the optimal C content. Two important MCE parameters were measured, namely, the...