Petrolia, Ont, incorporated as a town in 1874, population 5528 (2011c), 5222 (2006c). The Town of Petrolia is located 30 km southeast of SARNIA, and west of London.

Petrolia was originally called Petrolea but its name was changed due to a clerical error. Both its original and current names reflect the discovery of oil in the area. Oil was discovered in 1861 although it was not until 1866 that it was developed. That same year it was incorporated as a village. The town experienced the first oil boom in North America. It became very prosperous and has for a time the highest per capita income in Canada. Evidence of this wealth can still be seen in the town's impressive public buildings. When development of the oil was hampered by the lack of a railway, the townspeople built their own, which paid for itself in the first 6 months of operation.

In the 1860s and 1870s several refineries were built in Petrolia, but in 1898 Standard Oil bought a controlling interest in Imperial Oil and moved Imperial's refinery to Sarnia. Thereafter, Petrolia declined as a refining centre and most of its oil was shipped by pipeline to Sarnia. A local attraction is the operating 19th-century oil field at the Petrolia Discovery site.