The folding of a pH-sensitive leucine zipper, i.e., a GCN4 mutant containing eight glutamic acid residues, has been investigated. A pH-jump induced by a caged proton (o-nitrobenzaldehyde, oNBA) is employed to initiate the process, and time-resolved IR spectroscopy of the amide I band to probe it. The experiment has been carefully designed to minimize the buffer capacity of the sample solution so that a large pH jump can be achieved, leading to a transition from a completely unfolded to a completely folded state with a single laser shot. In order to eliminate the otherwise rate-limiting diffusion-controlled step of the association of two peptides, they have been covalently linked. The results for the folding kinetics of the cross-linked peptide are furthermore compared to those of an unlinked peptide, which reveals a detailed picture of the folding mechanism. That is, folding occurs in two steps, one on a ~1-2 us timescale leading to a partially folded alpha-helix even in the monomeric case, and a second one leading to the final coiled-coil structure on distinctively different timescales of ~30 us for the cross-linked peptide and ~200 us for the unlinked peptide. By varying the initial pH, it is found that the folding mechanism is consistent with a thermodynamic two-state model, despite the fact that a transient intermediate is observed in the kinetic experiment.