isologous

from The Century Dictionary.

• Having similar proportions or relations: specifically applied in chemistry to a series of hydrocarbons each member of which differs in composition from the next above it in the same series by having two less hydrogen atoms. Thus, ethane (C₂H₆), ethylene (C₂H₄), and acetylene (C₂H₂) form an isologous series.

from the GNU version of the Collaborative International Dictionary of English.

• adjective (Chem.) Having similar proportions, similar relations, or similar differences of composition; -- said specifically of groups or series which differ by a constant difference.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective chemistry Having similar proportions, similar relations, or similar differences of composition; said specifically of groups or series which differ by a constant difference.