Two chemical terms (3,641 and 1,913 letters long) have appeared in the Guinness Book of World Records. They were withdrawn because they have never been used by chemists, and there is no theoretical limit to the length of possible legitimate chemical terms. A DNA molecule could have a name of over 1,000,000,000 letters if it was written out in full.

(1,185) ACETYL­SERYL­TYROSYL­SERYL­ISO­LEUCYL­THREONYL­SERYL­PROLYL­SERYL­GLUTAMINYL­PHENYL­ALANYL­VALYL­PHENYL­ALANYL­LEUCYL­SERYL­SERYL­VALYL­TRYPTOPHYL­ALANYL­ASPARTYL­PROLYL­ISOLEUCYL­GLUTAMYL­LEUCYL­LEUCYL­ASPARAGINYL­VALYL­CYSTEINYL­THREONYL­SERYL­SERYL­LEUCYL­GLYCYL­ASPARAGINYL­GLUTAMINYL­PHENYL­ALANYL­GLUTAMINYL­THREONYL­GLUTAMINYL­GLUTAMINYL­ALANYL­ARGINYL­THREONYL­THREONYL­GLUTAMINYL­VALYL­GLUTAMINYL­GLUTAMINYL­PHENYL­ALANYL­SERYL­GLUTAMINYL­VALYL­TRYPTOPHYL­LYSYL­PROLYL­PHENYL­ALANYL­PROLYL­GLUTAMINYL­SERYL­THREONYL­VALYL­ARGINYL­PHENYL­ALANYL­PROLYL­GLYCYL­ASPARTYL­VALYL­TYROSYL­LYSYL­VALYL­TYROSYL­ARGINYL­TYROSYL­ASPARAGINYL­ALANYL­VALYL­LEUCYL­ASPARTYL­PROLYL­LEUCYL­ISOLEUCYL­THREONYL­ALANYL­LEUCYL­LEUCYL­GLYCYL­THREONYL­PHENYL­ALANYL­ASPARTYL­THREONYL­ARGINYL­ASPARAGINYL­ARGINYL­ISOLEUCYL­ISOLEUCYL­GLUTAMYL­VALYL­GLUTAMYL­ASPARAGINYL­GLUTAMINYL­GLUTAMINYL­SERYL­PROLYL­THREONYL­THREONYL­ALANYL­GLUTAMYL­THREONYL­LEUCYL­ASPARTYL­ALANYL­THREONYL­ARGINYL­ARGINYL­VALYL­ASPARTYL­ASPARTYL­ALANYL­THREONYL­VALYL­ALANYL­ISOLEUCYL­ARGINYL­SERYL­ALANYL­ASPARAGINYL­ISOLEUCYL­ASPARAGINYL­LEUCYL­VALYL­ASPARAGINYL­GLUTAMYL­LEUCYL­VALYL­ARGINYL­GLYCYL­THREONYL­GLYCYL­LEUCYL­TYROSYL­ASPARAGINYL­GLUTAMINYL­ASPARAGINYL­THREONYL­PHENYL­ALANYL­GLUTAMYL­SERYL­METHIONYL­SERYL­GLYCYL­LEUCYL­VALYL­TRYPTOPHYL­THREONYL­SERYL­ALANYL­PROLYL­ALANYL­SERINE = Tobacco Mosaic Virus, Dahlemense Strain.
This word has appeared in the American Chemical Society's Chemical Abstracts and is thus considered by some to be the longest real word.

(39) TETRA­METHYL­DIAMINO­BENZHYDRYL­PHOSPHINOUS = a type of acid.
This is the longest chemical term in the Oxford English Dictionary (2nd Ed.). It does not have its own entry but appears under a citation for another word.

(37) FORMALDEHYDE­TETRA­METHYL­AMIDO­FLUORIMUM
Chemical term in the Oxford English Dictionary (2nd Ed.).

(37) DIMETHYL­AMIDO­PHENYL­DIMETHYL­PYRAZOLONE
Chemical term in the Oxford English Dictionary (2nd Ed.).

(31) DICHLORO­DIPHENYL­TRICHLORO­ETHANE = a pesticide used to kill lice; abbrv. DDT.
It is the longest word in the Macquarie Dictionary and is also in the Oxford English Dictionary (2nd Ed.).

(29) TRINITRO­PHENYL­METHYL­NITRAMINE = a type of explosive.
This is the longest chemical term in Webster's Dictionary (3rd Ed.).

(27) ETHYLENE­DIAMINE­TETRA­ACETATE
The longest unhyphenated word in Merriam-Webster's Collegiate Dictionary (10th Ed.), joint with electroencephalographically (see above).

(26) ETHYLENE­DIAMINE­TETRA­ACETIC = a type of acid; abbrv. EDTA.
This word appears in Merriam-Webster's Collegiate Dictionary (10th Ed.).