Adenine (HAd) reacts with CH3HgX (X = NO3, ClO4) in basic aqueous solution to form a compound [CH3HgAd]•H2O, which crystallizes in the monoclinic space groupC2/c, witha = 19.796,b = 7.119,c = 16.471 Å, β = 128.83°, and Z = 8. The CH3Hg+group is linearly bonded to N(9) of deprotonated adenine and the molecules are held in pairs by twohydrogen bonds. With excess adenine, higher complexes of the formula [(CH3Hg)nAd]Xn−1(wheren = 2, 3) are formed, in which positions N(9) (deprotonated), N(7), and N(3) are successively filled. Compounds [(CH3Hg)3(Ad-H)]X have been obtained at high pH by substitution of H(9) and an amino hydrogen, and with coordination of a third CH3Hg+group probably to N(7). Complexation leads to significant changes in the infrared spectra and the 1750–1250 cm−1regions are correlated with the substitution patterns. The structures are discussed in connection with the basicity of the donors in adenine and the basicity changes of the remaining donors when other atoms are already involved in coordination.