The effect of carburization on the impact strength and microstructure of the commercial Fe-32 Ni-20 Cr alloy 800H has been investigated in the 20 to 800°C temperature range. The properties and microstructure of test pieces carburized at 850°C for up to 500 h in an argon-10 vol% methane atmosphere and of specimens heat treated at 850°C in an inert atmosphere for the same times were compared. The results showed that aging at 850°C reduced the impact strength at 20 to 800°C. With an increasing degree of carburization, the impact strength was progressively reduced to ∼50 J at a bulk carbon content of 0.6 wt%. Heat treatment after carburization caused a further decrease in impact strength as the depth of carbon penetration increased. Microstructural examination by optical and transmission electron microscopy (TEM) of broken test specimens showed precipitation of M23C6 carbides on grain and twin boundaries and intragranular fine precipitation of TiC and M23C6 as well as the presence of primary titanium carbonitrides. The TEM investigations using extraction replica and thin foil techniques established that the M23C6 carbides at grain boundaries retained a crystallographic orientation to one grain and grew into the adjacent grain. Lamellae of M23C6 carbides precipitated on noncoherent twin boundaries grew into the grain parallel to the twin plane, whereas M23C6 on coherent twin planes grew as plates along the twinning plane.