The effect of halide ions (F-, Cl-, Br-, I-) on nickel deposition in acidic electroless nickel plating baths is investigated. Halide ions were found to have a significant effect on the nickel deposition and the results could not fully be explained using mixed-potential theory. A correlation between the stability constants of halide ions with palladium (II) ions and the plating rate is proposed to explain the observations. Various parameters, such as the activation energy, deposit microstructure and phosphorus contents of the plating bath in the presence of various halide ions, were also studied.