In January and February 1937 she visited Málaga, together with Hungarian journalist and reporter for the British daily newspaper "News Chronicle", Arthur Koestler. She and Koestler took shelter with the eccentric 72 year old Sir Peter Chalmers Mitchell, who had stayed on in Málaga "to protect his house and servants" while his compatriots fled to Gibraltar. During the battle of Málaga she barely escaped the attacking Nationalist forces. Grepp left Málaga on 6 February, while Koestler was still in the city. On 7 February Italian troops occupied the city. Koestler was arrested, sentenced to death as a spy, and placed in a death cell in Sevilla. However, after considerable international pressure he was released from custody. From May 1937 Grepp spent several weeks in the Basque Country. She visited the Republican Basque Army defensive line called the Iron Belt, and experienced the Battle of Bilbao. Grepp frequently found herself in dangerous situations while in Spain. During her time in Spain Grepp was suffering from tuberculosis. Eventually she was compelled by her ill health to leave the war zone and return to Norway.