A new ligand 2-hydroxybenzaldehyde[(1Z,2E)-2-(hydroxyimino)-1-methyl propylidine] hydrazone (HBHMPH) synthesised and reported for the first time in this work has been studied potentiometricaly in mixed solvent system (Dioxane + water ) at various ionic strengths to obtain the acidity constant of the ligand. Further, due to the perceived interests in the coordinating behaviour of this ligand with various transition metal ions, the potentiometric studies in solution similar to those for the ligand were carried out to obtain the metal-ligand formation constants. The studies revealed that the ligand behaves a monobasic acid in spite of two probable dissociable protons being present in it viz. the oximino proton and the phenolic proton. The nature of this monobasic character is studied. The metal complexes of Co (II), Ni (II), Cu (II), and Zn (II) show that the most stable species in the ML2 complex.