To provide a framework for discussing why different solvents act the way they do, generalized molecular structures of two important solvent families used in lacquer thinners are described. The hydrocarbon family is made up of three main groups - aliphatic, aromatic and chlorinated hydrocarbons. Saturated aliphatic hydrocarbons have groups of molecules that have only C-C single bonds and no C=C double bonds. Examples such as hexane, heptane and octane are components of oil based reactive finishes such as polyurethanes. The cyclic aliphatic hydrocarbons are saturated hydrocarbons that have formed a ring or multiple ring structure. Cyclohexane and turpentine are examples. Aromatic hydrocarbons such as toluene, ethyl, xylene and diethylbenzene are based on the benzene molecule and are important components of some acrylic and nitrocellulose lacquers. Chlorinated hydrocarbons include methylene chloride which is the main ingredient in most solvent-type paint removers. They have one or more hydrogen atoms replaced by an atom from the halogen group. The second family of solvents, the oxygenated solvents, incorporates oxygen atoms into the hydrocarbon structure. This family includes esters, glycol ethers, ketones, and alcohols. Important solvent chemicals in these groups are ethyl acetate, methyl cellosolve , acetone, and methanol respectively. This is part 1 of a two part article on solvents and solutions.