1 : 1 Complexes of pyridine N-oxide, and 2-methyl, 3-methyl, and 4-methyl, 2,6-dimethyl, and 4-cyano pyridine N-oxides with mercury(II) chloride and bromide have been prepared and characterised by molecular conductance and i.r. spectral measurements (down to 222 cm.^–1). It is suggested that these complexes are dimeric tetrahedral structures with halogen bridges in the solid state. Assignments for the mercury(II)–oxygen and mercury (II)–halogen modes have also been made.