In order to study the effect of carbon impurities in plasmas on tungsten first walls, a hydrogen–carbon mixed ion beam was irradiated on tungsten. It was found that a very small amount of carbon (∼0.3% or more) significantly enhanced blister formation. Small blisters appeared in a fluence of ∼1023 H m−2 and their number increased in the 1024 H m−2 range. Then, the size of the blisters increased towards the 1025 H m−2 range. According to XPS analysis, all the tungsten atoms near the top surface were combined with carbon atoms to form a tungsten carbide layer in the cases where significant blisters were observed. This layer prevented the implanted H from leaving the tungsten. As a result, it caused enhanced diffusion into the bulk, leading to blister formation.