In regard to, the important role of the samarium and gadolinium in industry, and also their toxicity, these ions have been studied in this thesis and two potentiometric membrane sensors have been made for them. In the first part of this work, initial studies exhibit that 1,3-di(thiopheneimino)benzoic acid (DTBA) complexes selectively with samarium ions. Thus, this ligand was used as an ionophore in construction of an ion selective electrode based on PVC for samarium. This electrode displays a linear response over the concentration range of 5.0 × 10 -6 to 1.0 × 10 -1 M Sm(III) with a Nernstian slope of -19.6 mV per decade. The electrode has a detection limit of 3.1 × 10 -6 M and shows very fast response time ( 5 s). The sensor is highly selective for Sm(III) ions in the presence of many cations. This sensor was successfully applied as a probe in determination of samarium in binary mixtures, and in indirect determination of hyoscine and hematropine in their pharmaceutical formulation. In the second part of this work, a sensitive membrane electrode based on 4-methoxyphenylcyanamide (4-MeOpcyd) for determination of gadolinium was prepared. This electrode exhibits a linear response with a Nernstian slope of -19.5 mV per decade within the concentration range 1.0 × 10 -6 M to 1.0 × 10 -2 M Gd(III). Detection limit of 6.2 × 10 -7 M and response time lower than 10 s are the advantages of this electrode. The prepared electrode applied for determination of gadolinium as an indicator electrode in potentiometric titration with EDTA and also for fluoride ions in mouthwash samples with satisfactory results.