Fiber reaction: Here the dye mole- cules or ions do not lose all their solubiliz- ing groups after diffusion into the fibers, but in the correct conditions they react and attach themselves by covalent chemical bonds to the much larger fiber molecules, to form new colored derivatives of the fibers. The small number of dye solubiliz- ing groups is totally inadequate to cause the large new dye-fiber molecules to dis- solve in water.