The chemistry of an 667 Mo interstellar cloud was studied using 142 reactions for 40 species during the contraction under self gravity in two steps. At first the contraction is allowed without gas phase reactions untill certain optical depth is reached. Secondly, at this optical depth the chemical reactions are started for sufficient cycles in a time dependant scheme till only very small additionally changes in the abundances occur. The so obtained, relative abundances and coulmn densities for different species represent a continous function of the optical depths. The values arround τ=6.3 represent the observations for H2, H2+, H3+, OH, OH+, CH, CH+, CH2, CH2+, CH3+, H2O and H3O+. The region of τ between 1 and 5 i.e. of particle density between 4 102–6 103 is the preferable formation place for the majority of molecules.