Electroless plating rate measurements for some copper and nickel baths have been made by the coulostatic method. The reciprocal of polarization resistance Rp
, obtained by the analysis of the coulostatic decay curve, showed reproducible proportional relationship with the deposition rate for each bath. The proportional constant between them was dependent on the kind of bath but was independent of the electrode material used. The value of double layer capacity Cd
was almost constant for the same bath composition, temperature, electrode material and measuring conditions. Based on these electrochemical characteristics of electroless plating, a plating rate monitor which detects time constant (Cd×Rp
) of the coulostatic decay was constructed and evaluated with the production electroless copper bath. Results suggest that this monitor is useful for rapid estimation of the deposition rate for production baths.