Electroless nickel (EN) plating is a process in which Ni2+ ions are reduced by hydrogen atoms adsorbed at a fresh Ni surface. However, detaching of a handful of tiny Ni metal particles from a substrate causes the entrance of these particles into the plating solution. The metal particles offer very reactive surfaces for the reduction of Ni2+ ions, which in turn aggravates the detachment, causing a self-accelerated cycle. Eventually the plating solution will be subject to an overwhelming precipitation of Ni black. This paper proposes a one-dimensional diffusion model to explain the dependence of the bath stability on the plating time under different bath loadings. This mathematical model contains Vd, defined as the decomposition volume, a measure to judge chemical stability of a plating solution. To obtain Vd experimentally, a PdCl2 solution was purposely introduced into a model solution (the addition leads to immediate generation of metal particles) until the very moment of onset of massive deposition of colloidal Ni. The Vd data from the experiment were then used to perform simulation in order to complete the model proposed. Other than the effects of bath loadings and plating time, an adsorption model was also created to describe the temperature effect. To coordinate the adsorption model, l-cysteine was used as an adsorbate that plays a deactivation role. The under bump metallization process on patterned silicon wafers has been used to support the main theme of this study.