A homologous series is a sequence of organic compounds with the same functional group and similar chemical properties in which the members of the series differ by a CH2 unit. Compounds within a homologous series typically have a fixed set of functional groups that gives them similar chemical and physical properties. For example, the homologous series of alkane: CH4, C2H6, C3H8, etc. are homologous. They differ from each other by a CH2 unit. The characteristics of homologous series include:

• The functional group of the members of the homologous series is the same.
• The general formula is the same for all members.
• Members of homologous series have nearly identical chemical properties due to the same functional group.
• Members of homologous series have a general method of preparation in common.

The concept of homologous series was proposed in 1843 by the French chemist Charles Gerhardt. Homologous series are not unique to organic chemistry. Titanium, vanadium, and molybdenum oxides all form homologous series, called Magnéli phases, as do the silanes.