Dibasic esters (diesters) and their use in plasticizer compositions are generally disclosed. In some embodiments, the diesters are branched-chain esters of long-chain alkanedioic acids, such as octadecanedioic acid. In some embodiments, such plasticizer compositions are used to increase the plasticity of a polymer resin, such as a vinyl chloride resin or poly vinyl butyral. In some other embodiments, such plasticizer compositions are used to lower the glass transition temperature of a polymer resin. In some embodiments, at least a portion of the plasticizer is derived from a renewable source, such as a natural oil.