William S. Benedict, 70, an authority on spectroscopy and a professor emeritus of the University of Maryland, died Thursday at Greater Southeast Community Hospital after a heart attack.

He was a research professor in the Institute for Molecular Physics at the University from 1967 until he retired last year.

Dr. Benedict was considered an authority on spectrum water vapor and a leader in the interpretation of the spectra of solar and planetary atmospheres. He was acclaimed for his fundamental contributions on the mechanism of the water vapor laser and for discovery of hydrogen chloride on Venus.

He came to Washington in 1942 and was a physical chemist at the Geophysical Laboratory of the Carnegie Institution for four years. He then served for six years as a senior physicist with the National Bureau of Standards.

From 1952 until joining the University of Maryland in 1967, Dr. Benedict was research contract director at Johns Hopkins University.

He was born in Lake Linden, Mich. He earned bachelor's and master's degrees from Cornell University and a doctorate from the Massachusetts Institute of Technology. He was a research fellow at Princeton University in 1933-35 and then a research chemist with the General Chemical Co. in New York before coming here.

Dr. Benedict, who lived in Washington, was a fellow of the American Physical Society and a member of the American Association for the Advancement of Science, the Washington Academy of Sciences, the Optical Society of America, the Societe Royal de Sciences in Liege, Belgium, and Phi Beta Kappa.

He was the author of numerous publications in his field.

He is survived by his wife, Dr. Ruth B. bEnedict of Washington; a son, Dr. Philip J., of Providence, R.I.; and one grandchild.