Soyo (formerly known as Santo Antonio do Zaire) is a city located in the province of Zaire in Angola. Soyo recently became the largest oil-producing region in the country, with an estimate of 1,200,000 barrels per day (190,000 m3/d).
Soyo (originally spelled "Sonho" and pronounced Sonyo) was a province of the Kingdom of Kongo, which stretched south from the mouth of the Congo River to the River Loze, and inland from about 100 kilometers. It was already an administrative entity whose ruler or governor bore the title mwene Soyo or "lord of Soyo when the Portuguese arrived in 1482. The ruler was the first Kongo lord to be baptized when Christian missionaries came to the kingdom of Kongo in 1491.
Soyo was typically ruled in the sixteenth century by a member of Kongo's royal family, presumably appointed by the king and serving for a limited term. The ruler at the time of the Portuguese, baptized as Manuel, was said to be the uncle of the ruling king. Within Kongo's oversight, Soyo was permitted to expand and conquer other regions under royal rule. Thus, Nzinga a Nkuwu, ruling king of Kongo in 1491 permitted an expansion of Soyo's territory following the baptism of the ruler. This expansion allowed Soyo to control several sub-provinces including Pambala, Kimi, Tubii, along the Congo River, and Lovata (among others) along the Atlantic coast.
Soyo's port of Mpinda, located near the mouth of the Congo River, became an important port in the sixteenth century trade of Kongo. A community of Portuguese settled there and conducted a trade in slaves, ivory and copper from the port. A Kongo royal inquest of 1548 revealed that as many as 4,000 slaves passed through Mpinda en route to the island colony of São Tomé, and then to Brazil every year.