Crystals of 7-azaindole ([Formula: see text], a = 11.312(4), b = 14.960(6), c = 15.509(5) Å, α = 102.86(3), β = 108.78(3), γ = 90.71(3)°, Z = 16, R = 0.052) contain tetrameric units of approximate S4 symmetry, in which the molecules are associated by means of four complementary N—H … N hydrogen bonds. [CH3Hg(7-azaindole)]NO3 ([Formula: see text], a = 7.818(3), b = 7.884(3), c = 9.135(4) Å, α = 97.89(3), β = 109.13(3), γ = 103.28(3)°, Z = 2, R = 0.039) contains well-separated nitrate ions and complex cations in which the methylmercury group is linearly bonded to the pyridine nitrogen atom, whereas the five-membered ring remains protonated. In the neutral [CH3Hg(azaindolate)] complex ([Formula: see text], a = 10.926(10), b = 11.333(8), c = 11.647(10) Å, α = 92.13(8), β = 104.83(9), γ = 111.86(7)°, Z = 6, R = 0.048), methylmercury groups have substituted the N—H proton in the five-membered ring for the three symmetry-independent molecules. Intermolecular secondary Hg … N bonds are found with pyridine nitrogens. Keywords: azaindole, methylmercury, crystal structure.