New Zealand-born American chemist who, with Alan J. Heeger (Heeger, Alan J.) and Shirakawa Hideki, was awarded the Nobel Prize for Chemistry in 2000 for their discovery that certain plastics can be chemically modified to conduct electricity almost as readily as metals.

MacDiarmid earned Ph.D.'s in chemistry at the University of Wisconsin at Madison (1953) and the University of Cambridge (1955). He then joined the faculty of the University of Pennsylvania, becoming full professor in 1964 and Blanchard Professor of Chemistry in 1988.

During a visit to Japan in the mid-1970s, MacDiarmid met Shirakawa, who reported that he and his colleagues had synthesized polyacetylene, a polymer that was known to exist as a black powder, into a metallic-looking material that still behaved as an insulator. In 1977 the two men and Heeger, collaborating at the University of Pennsylvania, decided to introduce impurities into the polymer much as in the doping process used to tailor the conductive properties of semiconductors. Doping with iodine increased polyacetylene's electrical conductivity by a factor of 10 million, which made it as conductive as some metals. The discovery led scientists to uncover other conductive polymers. These polymers contributed to the emerging field of molecular electronics and were predicted to find application in computers.

MacDiarmid held some 20 patents and was the recipient of numerous awards. In 2001 he was made a member of the Order of New Zealand, the country's highest honour.