Abstract. Mr=578.59 , triclinic, P]-, a=8.701 (6), b=9.127(2), c=15.043(2) A, tx=82.08(2), fl= 88.86 (4), y--78.15 (4) °, V= 1158.0/~3, Z --4, D_ = 3.318, D m = 3-32 Mg m -3, 2(Mo Ka) = 0.71069/( (graphite-monochromated), p = 26.45 mm -~, F(000) = 1016, T= 293 K, final R--4.4% for 2553 unique observed reflections. The asymmetric unit consists of two independent molecules, in which both NH 2 protons of 9-methyladenine have been substituted by linearly coordinated methylmercury groups. The--N(HgCH3) 2 group is approximately in the plane of the ring.