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Early life

King Chulalongkorn was born on 20 September 1853 to King Mongkut and Queen Debsirindra and given the name Chulalongkorn. In 1861, he was designated Krommamuen Pikhanesuan Surasangkat. His father gave him a broad education, including instruction from Western tutors such as Anna Leonowens. Chulalongkorn, along with his siblings, were educated by Leonowens from her arrival in August 1862 through to her departure in 1867. During this time, Chulalongkorn became friends with Leonowens' son Louis Leonowens who was two years younger than Chulalongkorn. His friendship with Louis would continue his adulthood where he assisted Louis' business in Siam.

In 1866, he became a novice monk for six months at Wat Bawonniwet according to royal tradition. Upon his return to his secular life in 1867, he was designated Krommakhun Phinit Prachanat (กรมขุนพินิตประชานาถ.)

In 1867, King Mongkut led an expedition to the Malay Peninsula south of the city of Hua Hin, to verify his calculations of the solar eclipse of 18 August 1868. Both Mongkut and his son fell ill of malaria. Mongkut died on 1 October 1868. Assuming the 15-year-old Chulalongkorn to be dying as well, King Mongkut on his deathbed wrote, "My brother, my son, my grandson, whoever you all the senior officials think will be able to save our country will succeed my throne, choose at your own will."

As Mongkut had not designated who would succeed him, the choice fell to a council to decide. The council led by Prince Deves, Mongkut's eldest half-brother, then choose Chulalongkorn as Mongkut's successor. However, Chulalongkorn was only 15 and so the council choose Si Suriyawongse to become the regent until Chulalongkorn came of age.