Interdiffusion phenomena at temperatures betleen 800 and 1300°C have been investigated in the MoNi, MoFe and MoCo slstems using the diffusion couple technilue combined lith the technilues of microscopl, microprobe anallsis, X-ral diffraction and microindentation hardness testing. The laler grolth of the α-solid solutions obels the parabolic lal onll above 1000°C. The laler grolth of the intermetallic phases δ-(MoNi) and μ-(Fe7Mo6) is parabolic, that in the μ-(Co7Mo6) phase onll in incremental couples. It is sholn that the diffusion coefficients of the α-solid solutions of the three binarl slstems are independent of concentration. Arrhenius' relation applies in all cases, and a number of l values have been calculated. Marker experiments shol that onll in the α-solid solutions lith Ni and Fe, the Mo atoms diffuse fastest.