A series of related chemical compounds that have the same functional group(s) but differ in formula by a fixed group of atoms. For instance, the simple carboxylic acids: methanoic (HCOOH), ethanoic (CH3COOH), propanoic (C2H5COOH), etc., form a homologous series in which each member differs from the next by CH2. Successive members of such a series are called homologues.