Department of Chemistry, University College, Thiruvananthapuram-635 034, India Department of Chemistry, St. Gregorios College, Kottarakara-691 531, India Manuscript received 21 June 2004, revised 11 April 2005, accepted 14 July 2005 Some new NiII complexes of the ligands 2,3-dimethyl-1-phenyl-4-(2-hydroxy-5-methyl phenyl azo)pyrazol-5-one (CAAPH) and 2,3-dimethyl-1-phenyl-4-(2-hydroxy-5-chloro phenyl azo)pyrazol-5-one (CPAAPH) having the compositions [Ni(LH)2Cl2], [Ni(LH)(Ac)2], [Ni(LH)(NO3)2], [Ni(LH)SO4], [Ni(LH)2(ClO4)]ClO4, [Ni(LH)2(NCS)CI], [Ni(L'H)2CI2], [Ni(L'H)(Ac)2], [Ni(L'H)(NO3)2], [Ni(L'H)SO4], (Ni(L'H)2(ClO4)]ClO4 and [Ni(L'H)2(NCS)CI] (LH = CAAPH and L'H = CPAAPH) have been synthesised and characterised by elemental analysis, IR, electronic and 1H NMR spectra, magnetic and conductance measurements, thermal and cyclic voltammetric studies. The ligands and complexes have been screened for their antibacterial activity against gram positive and gram negative bacteria.