We present a study of the s–process nucleosynthesis (weak s-process) occurring during convective core He–burning and convective shell Carbon–burning in a massive star of 25 M$_{\odot}$. We use an updated nuclear network for the various neutron sources and for all neutron captures and β-decay rates involved. Large uncertainties affect the final yields due to the present unsatisfactory knowledge of all neutron capture cross sections involved.