In this diploma thesis, the electrochemical properties of vitamin B12 were determined by cyclic and differential pulse voltammetry. The aim of this study was to investigate the influence of pH, scan rate and concentration of vitamin B12 on its oxido-reduction properties, to study the possible adsorption of vitamin B12 on the working electrode surface and to detect vitamin B12 in a real sample. Electrochemical measurements were performed in a three-electrode electrochemical cell with a working glassy carbon electrode, reference Ag/AgCl electrode, and platinum wire counter electrode. The cyclic voltammograms of vitamin B12 have shown one oxidation peak which corresponds to the oxidation of Co2+ to Co3+ and one reduction peak which corresponds to a reduction of Co3+ to Co2+. It was determined that the oxidation of Co2+ to Co3+ is an adsorption-controlled process, while the reduction of Co3+ to Co2+ is a diffusion-controlled process, because the logarithm of the anodic and cathodic current peaks is a linear function of the logarithm of the electrode polarization speed (the slope of the line is about 1 for the anodic and about 0.5 for the cathodic peak current). Differential pulse voltammograms have shown that the anodic peak current of vitamin B12 was a linear function of concentration in a concentration range from 1.0·10 -6 mol dm-3 till 1.2 ·10 -4 mol dm-3. Differential pulse voltammetry was used to detect Vitamin B12 in a real sample of B-complex capsules.