The effects of Al, Ti, Bi, Te and S on the formation of hydrogen blowholes have been investigated by the technique of solidifying molten Fe-C-Si alloys in quartz tubes. This technique was employed to control the hydrogen content in the melts immediatately before solidifying and to eliminate the influence of mold conditions. Results obtained are as follows ; (1) The Al addition had no influence on the blowhole formation in white iron. On the other hand, the Al addition reduced the tendency of blowhole formation in gray iron. (2) The addition of Ti had no influence on the blowhole formation in white iron. (3) In the case of gray iron with Ti contents in the 0.091 to 0.108%, intermediate Al contents of 0.046% to 0.124% increased the tendency of blowhole formation and Al contents over 0.230% had no influence on the blowhole formation as compared with Ti-free samples. (4) The addition of Bi, Te and S increased the tendency of blowhole formation in white iron.