In April 1860, Loch accompanied Lord Elgin to China again, as secretary of the new embassy sent to secure the execution by the Chinese Qing Empire of its treaty engagements. The embassy was backed up by an allied Anglo-French force. With Harry Smith Parkes he negotiated the surrender of the Taku Forts (Dagu Forts). During the advance on the Qing capital, Peking (Beijing), Loch was chosen with Parkes to complete the preliminary negotiations for peace at Tungchow (present-day Tongzhou District, Beijing). They were accompanied by a small party of officers and Sikhs. It having been discovered that the Chinese were planning an attack on the British force, Loch rode back and warned the outposts. He then returned to Parkes and his party under a flag of truce in the hope of securing their safety. However, they were all taken prisoner by the Qing general Sengge Rinchen and incarcerated in the Ministry of Justice (or Board of Punishments) in Beijing, where the majority of the group died from torture or disease. Parkes and Loch were treated less barbarically after Prince Gong intervened. After three weeks, the negotiations for their release were successful, but they had only been liberated ten minutes when orders were received from the Xianfeng Emperor, who was then taking shelter in the Chengde Summer Palace, for their immediate execution.