Thermal behaviour of a number of organomercury(II) complexes of the type, p-XC6H4HgCl(L1) (I), p-XC6H4HgCl3(L2) (II), p-XC6H4HgL3 (III) and p-XC6H4HgL4 (IV) [L1=isoniazid, L2=theobromine, L3=phenyldithiocarbamate, L4=p-nitrophenyldithiocarbamate; X=Me, MeO, NO2] has been investigated. From TG curves, the order and activation energy of the thermal decomposition reaction have been elucidated. The variation of the activation energy has been correlated with the nature of the substituent on the phenyl ring. The heat of reaction has been elucidated from DSC or DTA studies. The fragmentation pattern has been analysed on the basis of mass spectra.