John J. Callahan, a retired Edgewood Arsenal chemist, died Wednesday of cancer at the Stella Maris Hospice. He was 66 and lived on Cider Mill Road in Parkville.

Services for Mr. Callahan were being held today at the Leonard J. Ruck Funeral Home, 5305 Harford Road.

He ended a 34-year career at the arsenal in 1985 as civilian chief of the Chemical Corps' Physical Organic Branch. He was an expert on the physical and thermal properties of materials and their rates of chemical action.

A native of Taylor, Pa., Mr. Callahan worked briefly at the Glenn L. Martin Co. after his graduation from high school.

In addition to his wife of 38 years, the former Gladys J. Spangler, his survivors include four sisters, Edith King and Ruth Skopyak, both of Somerset, N.J., Jean Boyd of Dallas, Pa., and Sarah Farrell of Highland Park, N.J.; three brothers, Joseph Callahan of North Brunswick, N.J., Leo Callahan of Scranton, Pa., and Walter Callahan of Taylor; and many nieces and nephews.

The family suggested memorial contributions to the Baltimore Society of the Archaeological Institute of America, 28 Thornhill Road, Lutherville 21093.