A series of organic compounds in which hydrogen in a carbon chain is replaced by the same functional group, is called homologous series. Any two adjacent homologues differ by (-CH₂) in their molecular formulae.

All the compounds of a homologous series show similar chemical properties.

Example of homologous series: Alkynes.

All the members of homologous series of alkynes have similar structure and similar chemical properties, so they can be grouped together into the homologous series. The general formula of homologous series of alkynes is C_nH_2n-2. Where, n is the number of carbon atoms in alkyne molecule. The members of alkyne homologous series are:

� Ethyne (C₂H₂)-First member of alkyne homologous series.

� Propyne (C₃H₄)-Second member of alkyne homologous series.

� Butyne (C₄H₆)-Third member of alkyne homologous series.

� Pentyne (C₅H₈)-Four member of alkyne homologous series.

� Hexyne (C₆H₁₀)-Five member of alkyne homologous series.