In this article, we discuss the phantom exposure of chemically amplified resists in x-ray lithography, which is attributable to acids in environmental air induced by x-ray irradiation. Since the acids generated in the air diffuse into the resist film from the top to the bottom like so many catalytic acids, they cause cross linking in a negative resist and decomposition in a positive resist. The phantom exposure in flood exposures of a 20 mm square increases as the exposure dose increases, and expands more than 10 mm outside the exposure field when the exposure dose is ten times greater than the optimum exposure dose for patterning. Dependence of the phantom exposure on the exposure dose and resist material are evaluated.