The adsorption and phase formation of bromide ions on Au(100) has been studied with in-site surface X-ray diffraction in combination with electrochemical measurements. The investigations allowed a rather detailed assignment of all voltammetric features of the bromide adsorption on Au(100) to the respective structural properties, their stability, and their transition kinetics. The presence of a small bromide surface excess at Au(100)-(hex) causes the lifting of the substrate surface reconstruction. The corresponding charge density just before the onset of the transition ranges from -2 to -8 mu C cm(-2). No ordering of the adlayer is required or involved. The lifting of the substrate reconstruction proceeds at high overpotentials via instantaneous nucleation and two-dimensional growth. The growth mechanism seems to change at low overpotentials toward a one-dimensional or surface diffusion controlled process. Two ordered bromide adlayer phases were found on the unreconstructed Au(100)-(1x1) surface. The low coverage phase (theta = 0.5) corresponds to a commensurate c(root 2x2 root/2)R45 degrees superstructure. A uniaxial commensurate c(root 2xp)R45 degrees phase is formed at more positive potentials, in which the coverage increases from 0.5 to approximately 0.565 (relative to the gold layer density) with increasing electrode potential. Current vs time measurements revealed that the transition between the disordered fluidlike phase and the commensurate c(root 2x2 root 2)R45 degrees phase proceeds via instantaneous islands (hole) nucleation and activation-controlled growth in combination with a parallel adsorption (desorption) process. The formation of the slightly more dense uniaxial commensurate c(root 2xp)R45 degrees adlayer can be described with the exponential law of nucleation in combination with either one-dimensional activation-controlled or two-dimensional surface diffusion controlled growth.