Donald J. Alderson, 55, a foreign affairs specialist wiht the Library of Congress, died of cancer Tuesday at Fairfax Hospital.

He had joined the library staff about five years ago after working for the Institute for Defense Analysis.

Born in Albany, N.Y., Mr. Alderson graduated magna cum laude from Harvard University. He served in World War II in Europe as an Army captain in the infantry and was awarded the Bronze Star.

After the war, he earned a master's degree in Hispanic-American studies at Stanford University and later another master's degree in political science at Yale University.

Mr. Alderson taught at Stanford and the University of Arizona. He was associated at one time with the University of Oklahoma international training program in Mexico.

His last position before coming here 15 years ago was with the Research Triangle Institute in North Carolina.

He was a member of the Veterans of the 7th Infantry, the Company of Military Historians and the American Society of Military Insignia Collectors.

He is survived by his wife, Janet C., of the home in Springfield, Va.; two sons, Stanley, of San Diego, and Douglas, of Springfield; a daughter, Barbara Alderson, of New York City, and his father, Dr. Stanley E. Alderson, and a sister, Constance Fite, both of Albany.