Plating rates and Ni co-deposition were measured for electroless Cu plating baths without and with an added proprietary stabilizer system. The bath has formaldehyde as reducing agent and tartrate as complexing agent. The concentrations of Ni and stabilizer codetermine the plating rate. We investigated this bath over a wide range of compositions, varying the concentrations of stabilizer, Ni, formaldehyde and hydroxyl ions. Stabilized baths without Ni plate very slowly, while with the addition of Ni plating rates are comparable to non stabilized baths. The amount of Ni in the film is proportional to Ni in the bath for stabilizer-free baths, but becomes dependent on plating rate when.the bath is stabilized. Extended absorption fine structure measurements show that the co-deposited Ni is metallic. A quantitative model for time-dependent plating kinetics and Ni incorporation is proposed. The model is based on the inhibition of plating by the adsorption of non-stabilizer and stabilizer species on the plating surface, and it is used to fit plating rates and Ni incorporation for 254 experimental combinations of chemical composition and plating times. (C) 2017 Elsevier B.V. All rights reserved.