Pd-Sn catalysts possess excellent catalyst performance for electroless metal deposition. They have attracted interest for converting non-conductive surfaces into conductive ones. The present investigation was undertaken to verify the decomposition behavior of a Pd-Sn catalyst solution from the viewpoint of the colloid and solution chemistries. HCl gas is volatilized during spontaneous decomposition of Pd-Sn catalysts, which suggests that the stability of Pd-Sn catalysts is related to stannous ion complexation, depending on the chloride ion concentration. The core of the Pd-Sn catalyst is protected with（SnCl3）－ molecules, which have a lone pair of electrons, to prevent the primary particles in the Cl－ concentration range of about 3 M or more from aggregating. The solution chemistry of stannous chloro-complexes shows that, in such a higher concentration of Cl－, a small number of （SnCl2）molecules with unoccupied p-orbitals exists in the Pd-Sn solution. Such molecules act as a cross-linking agent. They enhance the probability of the aggregation of primary particles. In general, it is believed from the results of aerated accelerating-tests that Sn2＋ ions are oxidized to Sn4＋ ions by dissolved oxygen molecules, and that the decrease in Sn2＋ concentration causes aggregation（or decomposition）. This work confirmed that Cl－ concentrations influ ence the decomposition of Pd-Sn catalysts. We conclude that the spontaneous decomposition mechanism of Pd-Sn catalysts differs from the decomposition mechanism of Pd-Sn catalysts performed by the accelerating-tests.