Rex Marshall, a radio and television announcer who for 25 years was the commercial voice for the Reynolds Aluminum Company, died Wednesday at the Veterans Hospital in White River Junction, Vt. He lived in Woodstock, Vt., and was 64 years old.

Mr. Marshall had suffered a heart attack on Tuesday in his office at radio station WNHV in White River Junction. He had been the owner, president and general manager of the station since 1969. In the area covered by the station, he was known for his ''Breakfast at the Hanover Inn'' program.

He was also the announcer for Maxwell House Coffee, Auto Lite spark plugs and Esso gasoline. He had been the announcer on the early Jack Parr talk shows in 1957. He began his radio career in Boston in 1937, and was an original staff announcer for WPIX-TV in New York City. He covered the 1948 Republican national convention for the station and anchored the station's nightly ''10 O'Clock News.''

He was born in Pemberton, N.J., and grew up in Utica, N.Y. In World War II, he was a pilot-instructor in the Army Air Corps. He is survived by his wife, Barbara D. Marshall of Woodstock; two sons, Peter and Jeffrey, both of New York City, and two daughters, Pamela Boone of White River Junction and Jamie Marshall of Woodstock.