When Mongkut died, his eldest son, Chulalongkorn (Rama V, 1868-1910), a minor at the time, succeeded him. Under his father's direction, Chulalongkorn had received a thorough education by European tutors. During the regency that preceded his coming of age, the young king visited Java and India in order to witness European colonial administration. Thus he was the first Chakkri monarch to leave the country. At his coronation in 1873, he announced the abolition of the ancient practice of prostrating before the monarch, which he regarded as unsuitable for a modern nation. A number of reform decrees followed, designed to modernize the judiciary, state finances, and political structure. The reforms, however, provoked a revolt by conservatives under Prince Wichaichan in December 1874. Although the revolt was suppressed, it obliged Chulalongkorn to abandon "radicalism" and proceed more carefully with reforms. It was more than a decade before the king and his associates were in a position to enact more significant changes.

One of the most far-reaching of the later reforms was the abolition of slavery and the phrai corvee. Slavery was eliminated gradually, allowing considerable time for social and economic adaptation, and only disappeared in 1905. As a result of the introduction of a head tax paid in currency and a regular army manned by conscription, the corvee lost most of its function, and wage labor, often provided by Chinese immigrants, proved more efficient for public works projects. Likewise, the introduction of salaries for public officials eliminated the need for the sakdi na. These reforms wrought profound changes in Thai society.

In 1887 the king asked one of his princes, Devawongse, to initiate a study of European forms of government and how European institutions might be fruitfully adopted. The following year, the prince returned with a proposal for a cabinet government consisting of twelve functionally differentiated ministries. The king approved the plan, though several years passed before it could be fully implemented. In 1893 Prince Damrong Rajanubhab, acting as minister of interior, began an overhaul of Siam's antiquated provincial administration. The old semifeudal system in the outer provinces was gradually replaced by a centralized state administration. Under Damrong, the Ministry of Interior became immensely powerful and played a central role in national unification.

Like his father, Chulalongkorn fully appreciated the importance of education. He founded three schools on European lines for children of the royal family and government officials, including one for girls. Specialized schools were attached to government departments for the training of civil servants. Study abroad was encouraged, and promising civil servants and military officers were sent to Europe for further education. In 1891 Prince Damrong went to Europe to study modern systems of education. Upon his return he became head of the new Ministry of Public Instruction, though he was obliged to assume the Ministry of Interior post a year later.

The country's first railroads were built during Chulalongkorn's reign, and a line was completed between Bangkok and Ayutthaya in 1897. This was extended farther north to Lop Buri in 1901 and to Sawankhalok in 1909. A rail line built south to Phetchaburi by 1903 was eventually linked with British rail lines in peninsular Malaya.

Data as of September 1987