SnCl₂ aqueous solution including HCl is used as a sensitizer of two-step catalyzation (sensitization-activation) pretreatment for electroless metal plating onto non-conducting substrates. The sensitizer is inactivated by aging. In this study, the relationships among the Sn²⁺ concentration of sensitizer, amount of adsorbates formed on the substrates, induction period of electroless Ni-P deposition and aging time have been investigated quantitatively. As the aging proceeded, the Sn²⁺ concentration decreased, while the amount of adsorbates, including Sn species formed by sensitization, increased. With the decrease of Sn²⁺ concentration, the amount of Pd adsorbates formed during activation decreased, and the induction period was extended. An independent investigation using mixture solutions of SnCl₂ and SnCl₄ gave similar results for aging. These results show that the aging of sensitizer induces the decrease of divalent Sn species and the rise of tetravalent Sn species, which is inactive for the formation of Pd adsorbates. Thus, the aging decreases the Pd adsorbates and extends the induction period.