The high­est moun­tain of the East Antarc­ti­ca Ranges, near the coast of Queen Maud Land. Large moun­tain ris­ing to 3,425 m, stand­ing 4 mi SE of Mount Berg­ersen in the Sor Ron­dane Moun­tains. Mapped by Nor­we­gian car­tog­ra­phers in 1957 from air pho­tos tak­en by U.S. Navy, 1946–47, and named for Major Gun­nar Isach­sen, leader with Cap­tain Hjal­mar Riis­er-Larsen of the Nor­we­gian expe­di­tion 1930–31.