Thomas C. Lyons Jr., 48, a population expert with the Agency for International Development, died of cancer Sunday at the Washington Hospital Center.

Mr. Lyons joined AID in 1967 as a demographic analyst. He was stationed in Lagos, Nigeria, from 1975 to 1977 and in Nairobi, Kenya, from 1977 until earlier this year, when he returned to this country for reasons of health. At the time of his death, he was chief of the policy development division for the office of population at AID. He received the agency's Superior Honor Award for his work on population assistance programs.

Mr. Lyons was born in Oil City, Pa. He served in the Navy from 1950 to 1954 and then went to Ohio University, where he earned bachelor's and master's degrees. His career in the population field included work with the Bureau of International Labor Affairs, Planned Parenthood-World Population, and the Library of Congress before he joined AID. He moved to the Washington area in 1963.

He took courses in international affairs at Georgetown University and had lectured on population at Georgetown , the University of Virginia and Ohio University. He was a member of the American Political Science Association.

Mr. Lyons was a member of St. Matthias the Apostle Catholic Church in Lanham and a member of the Teams of Our Lady, a lay organization devoted to prayers and discussions in homes.

Survivors include his wife, the former Janet Betz, and four children, Michelle, John-Paul, Thomas C. III, and Ann Marie, all of the home in Lanham, and his parents, Mr. and Mrs. Thomas C. Lyons Sr., of Jacksonville, Fla.