In this project a hydrogen sulfide sensor is made in order to determine the concentration of H 2 S in methane gas. This sensor is a kind of electrochemical sensor composed of three electrodes: reference, counter, and working. All of them are made of platinum and H 2 SO 4 is used as an electrolyte. H 2 S gas can diffuse to the working electrode surface and it can be in touch with 2 phases of liquid and solid. The potential of working electrode is in the amount which H 2 S gas can be oxidized and changed into H 2 SO 4 . In this way a current will be produced which is proportional to H 2 S concentration. The linear range of the sensor i between 0.7 ppm to 43.2 ppm with a detection limit of (LOD) 0.7 ppm. This sensor has a high selectivity, sensitivity and a good response time. The oxidation of cysteine also has been studied at a glassy carbon electrode by electrocatalytic effects of Nile blue as a homogeneous mediator, using cyclic, linear sweep voltammetry and chronoamperometry as diagnostic techniques. The results showed that the catalytic current of the cysteine depends on the concentration of cysteine. Although cysteine itself showed a very poor electrochemical response at glassy carbon electrode, the response could be greatly enhanced by using Nile blue as a mediator, which enables a sensitive determination of cysteine. The catalytic rate constant was estimated for catalytic reaction of Nile blue with cysteine using chronoamperometry. The linear range is between 10 × 10 -6 to 25.0 × 10 -5 M with a detection limit of (LOD) 1.36 µM. The relative standard deviation (RSD%) for five replicated analysis of 5.0 ×10 -5 M is 1.1%. The influence of potential interfering substances on the peak current was studied and the results showed that the method is highly selective for cysteine determination. The proposed method was used for determination of cysteine in synthetic and real samples