SOLVENTS ARE SUBSTANCES, USUALLY LIQUIDS, which are capable of dissolving other substances to bring them into liquid form. In the paint and coating industry, solvents dissolve the solid or semisolid film-forming resins and reduce viscosity so that the paint can be applied as a uniform, thin film to a surface. Although solvents are transient components of a paint, they significantly affect not only the application characteristics of a paint, but also the appearance, physical properties, and durability of the coating. The two most important performance requirements that must be considered in selecting the proper solvent for any coating end use are solvency and evaporation rate. These key properties control initial paint viscosity during application, coating viscosity at various stages of drying, and final coating appearance. Solvents must evaporate relatively quickly during initial drying to prevent excessive flow and sag, but they must evaporate more slowly in the later stages to provide good leveling and adhesion. Solvency and evaporation rate are often measured indirectly since direct measurements are not always feasible or convenient. In addition, there are numerous other solvent properties that must be considered for specific applications. These are often listed as requirements in the solvent specifications and include measures of purity, uniformity, safety, and compliance with air pollution regulations. There are many different solvents used by the coatings industry. To facilitate their review and comparison, it is convenient to classify them chemically into three general categories: Hydrocarbon solvents, oxygenated solvents, and other solvents. Each category will be discussed separately in the sections that follow. Solvents may also be classified according to the function they perform: Active, latent, and diluent. An active solvent is a true solvent for the film-forming resin and has the major role in dissolving it. A latent solvent alone will not dissolve the resin, but behaves as an active solvent or has a synergistic effect when used in conjunction with an active solvent. A diluent usually has no solvency for the resin, but is tolerated by it in blends. Diluents are added to reduce cost and vehicle viscosity through dilution.