The SOIR instrument, flying on board Venus Express, operates in the infrared spectral domain and uses the solar occultation technique to determine the vertical profiles of several key constituents of the Venus atmosphere. The retrieval algorithm is based on the optimal estimation method, and solves the problem simultaneously on all spectra belonging to one occultation sequence. Vertical profiles of H2O, CO, HCl, and HF, as well as some of their isotopologues, are routinely obtained for altitudes ranging typically from 70 to 120 km, depending on the species and the spectral region recorded. In the case of CO2, a vertical profile from 70 up to 150 km can be obtained by combining different spectral intervals. Rotational temperature is also retrieved directly from the CO2 signature in the spectra. The present paper describes the method used to derive the above mentioned atmospheric quantities and temperature profiles. The method is applied on some retrieval cases illustrating the capabilities of the technique. More examples of results will be presented and discussed in a following companion paper which will focus on the CO2 vertical profiles of the whole data set.