It is well known that, during the widely used AAA pretreatment (de Vries and Barendsen 1954), alkali treatment is responsible for the incorporation of modem carbon due to the precipitation of atmospheric CO2 as carbonate. Until now, the last step of the experiment, consisting in acid treatment (most of the time with hydrochloric acid) was considered to be sufficient to eliminate all of lab contamination. But wood, peat and sediment present a complex molecular structure, During radiocarbon chemical treatments, functional groups still present in the molecules are likely to form ionic bonds with modern carbonates. These new chemical bonds resist a classical acid treatment and are responsible for rejuvenation. This short paper presents preliminary results for two common C-14 cases: rejuvenation of a 0.4 pMC wood and of an Oxygen Isotope Stage 3 (OIS3) paleosol. For both cases, contamination due to incorporation of modem carbon during chemical treatment is evaluated and an alternative protocol is proposed.