This paper is the first of a series of three articles in which we present the results and analyses of an extended study of the c-Si/Ti solid state reaction. In this paper we will discuss the spectroscopic ellipsometric investigation. Thin (≈10nm) Ti films are grown on clean Si(111) surfaces and are subsequently heated. The Si indiffusion and the Si-Ti intermixing are continuously registered by three-wavelengths ellipsometry. Two metastable intermediate phases are observed to form before the final state is obtained Spectroscopic ellipsometry (E = 2−4.5 eV) is used to characterize the as-deposited layer, the metastable intermediate phase and the final state. Analysis of these spectra shows that: (1) Si and Ti intermix during the initial Ti deposition, (2) a fast reordering of the Ti atoms occurs when the system is slightly heated (≈175°C), (3) a metastable, probably monosilicide phase with a large Si concentration gradient is obtained at ≈350°C, (4) a homogeneous metastable TiSi2 forms at ≈450°C, at ≈700°C a roughened TiSi2 layer with a surplus of c-Si is formed.