In this study, in-situ transmittance measurement was employed to study the stability and reactivity of the Cu electroless deposition (ELD) solution as an alternative tool for chemical-sensitive electrochemical analysis and ex-situ studies of deposited films. Its advantages lie in both the simplicity of the analysis and the in-situ allowance in time-dependent characterization. To understand the basic behaviors of Cu ELD solution, the change of transmittance with the size of Cu particles by the injection of SnPd colloids were observed. Based on the relationship between the transmittance and the Cu particle growth, in-situ monitoring was applied to determine the effect of complexing agents, the important elements in determination of solution performance, on stability and reactivity. In the stability test, it was observed that complexing agents with lower formation constant (pK(f)) led to the decomposition of solution at 70 degrees C, whereas complexing agents with higher pK(f) showed stable state. In reactivity test, the reaction time to Cu reduction with SnPd colloid was increased in the order of pK(f) values. Also, the feasibility of this in-situ monitoring method was confirmed by comparison with the results from real deposition on Ta substrate. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3606532] All rights reserved.