Department of Post-graduate Studies and Research in Chemistry, University of Mysore, Manasa Gangotri, Mysore-570 006 Manuscript received 18 December 1982, revised 12 December 1983, accepted 27 February 1984 Promazine hydrochloride (PMH) forms a red coloured complex with Pd(ll) in acetic acid-sodium acetate buffer of \(\mathcal{p}\)H 4.5-5.5 and a red coloured radical cation with Os(VIII) or Os(VI) in 1-3 M hydrochloric acid medium. The palladium-PMH complex and the radical cation exhibit maximum absorption at 498 and 515 nm, respectively.The composition of the Pd(II)-PMH complex is 1 : 1. The apparent stability constant of the complex at\(\mathcal{p}\)H 5.0±0.1, 27 ± 1o has a log K value. of 5.9 ± 0.1.The proposed methods offer the advantages of simplicity, stability, rapidity and sensitivity without the need for an extraction step or heating the solution.