Rates of detritiation of 5-X-2-tritiothiophens (X = CI, Br, I, Me, Bu^t, Ph, 2-thienyl, and 5-methyl-2-thienyl) in trifluoroacetic acid–acetic acid mixtures at 24·8° have been measured. The effects of the substituents generally parallel those of para-substituents in detritiation of substituted tritiobenzenes, and are in accord with the σ_p⁺- constants of the X groups, but 5-phenyl-2-tritiothiophen is rather less reactive than would be expected by comparison with 4-tritiobiphenyl. The activating effect of a 5′-methyl group on reaction at the 5-position of 2,2′-bithienyl agrees with that of a 4′-methyl group on reaction at the 4-position of biphenyl.