The induction period of the chemical reaction between Cu and S was visualized with STM, revealing a complex interplay between strain relief and reaction mechanism. Copper sulfide formation, monitored by high-resolution soft X-ray photoemission, started only after adsorption of similar to 0.2 ML of S on a two-atom thick Cu layer supported by a Ru(0001) substrate. The pre-sulfide stages of sulfur interaction were investigated by in situ STM. Sulfur decorated the Cu surface by adsorbing at pseudo it-fold hollow sites pf the striped Cu phase and restructured the Cu layer to create additional adsorption sites of this type. Several self-organized structures of adsorbed S on the dynamically responding strained Cu layer were observed. Significant structural changes of both adsorbate and adsorbent occurred within the induction period of this surface chemical reaction.