During dyeing processes, a high degree of exhaustion is desirable for economic and environmental reasons. Auxiliary chemicals are often added to the dyebath to improve exhaustion. The attraction of the dye to the fiber is called the affinity of the dye. The principal force behind the transfer of a dye molecule from the dyebath to the fiber is the concentration of the dye in the two phases. The relative amounts of dyebath and fiber used influence the exhaustion. Low liquor ratios, or a smaller amount of dyebath relative to the amount of fiber, provide higher dyebath exhaustion. Lonic and nonionic dyes are the two types of dyes that interact with water. Nonionic dyes are not strongly interactive with water and are usually used as dispersions in water. Hydrophilic fibers, such as cotton, rayon, and wool, attract water. Water molecules then diffuse into the amorphous regions of the fiber and break internal hydrogen bonds, causing fiber swelling, which can be enhanced by higher temperatures. Hydrophobic fibers, such as polyester, do not swell significantly in water. Dye and fiber interactions, ionic bonding, and covalent bonding are discussed.