Cysteine-based conjugation is one of the main antibody conjugation strategies for generating both heterogeneous and homogeneous Antibody Drug Conjugates (ADCs), being the reduction of the antibody a crucial step in these processes. In this work we have analysed the reduction conditions for the site-directed conjugation of an anti-HER2 (Human Epidermal Receptor 2) Trastuzumab_cys114 antibody to the cytotoxic drug vcMMAE (valine-citrulline monomethyl auristatin E), with an aimed average DAR (Drug-Antibody Ratio) of 2. Initial reduction was found to have a direct impact on the availability of free thiols for the conjugation: increasing of reducing agent concentration (until a molar excess of 50x) in the reduction step resulted in a lower proportion of naked antibody in the final ADC product and allowed us to obtain an ADC close to the DAR of interest. This work shows that reduction conditions must be adjusted in order to obtain the desired homogeneous ADC product