Phenothiazines substituted in the 2 and 10 positions exhibit many valuable analytical properties due to their characteristic structure (the presence of chemically active sulphur, nitrogen and substituents). They are easily oxidised in an acidic medium by some oxidising agents with the formation of coloured products. The first step is loss of one electron giving a coloured semiquinone free radical. This reaction is reversible. The free radical can lose another electron giving the colourless phenothiazonium ion. It hydrolyses to phenothiazine derivative S-oxide. This property has been exploited in spectrophotometric analysis. L-ascorbic acid is one of the basic vitamins, necessary to the normal growth and behaviour of organism. The most important chemical characteristic of L-ascorbic acid is its strong reducing power. Numerous scientists, both in vitro and in vivo investigated the mechanism of oxidation of C ascorbic acid. This compound has also many valuable analytical properties. The aim of this paper is to present the optimisation of the geometry of the product of reaction between phenothiazine derivative and L-ascorbic acid. Geometry optimisation calculates and displays a structure with a minimum energy and minimal atomic forces (gradient). The research was carried out using semiempirical CNDO calculations with minimisation based on Polak-Ribiere'a conjugate gradient method. The structures have been confirmed by spectrophotometry and voltammetry experiments.