China’s exports of a chemical used to improve the quality of gasoline surged as producers took advantage of higher prices overseas.

Exports of a group of chemicals that includes methyl tert-butyl ether, known as MTBE, rose to about 87,400 tons in May, according to government data. That’s a sixfold gain from April and the highest level in Chinese customs figures going back to 2017. Industry consultant OilChem estimates that most of the exported volumes were MTBE, which boosts the octane rating of gasoline.

Fuel markets have tightened as the peak summer demand period kicks off in the northern hemisphere, with supply in part crimped by upended trade flows following Russia’s invasion of Ukraine. The availability of components that are blended with gasoline have also been squeezed because of a lack of refining capacity across many regions. The US still makes and ships MTBE, but phased out its domestic use in the 2000s due to water contamination concerns.

Malaysia received the most flows from China last month followed by Singapore and Japan, according to OilChem. Unlike finished products such as gasoline and diesel, Chinese producers are not constrained by export quotas for MTBE. Trade data for chemical products is occasionally adjusted by the customs agency.

China’s MTBE exports to Europe and the Americas are estimated to reach about 50,000 tons over May and June, according to global commodity intelligence provider ICIS. The last shipments of scale from China outside of Asia was about 11,600 tons sent to Chile in 2019, said ICIS analyst Huang Lifang. Volumes flowing to Singapore could also be redirected to the west, according to ICIS.

Weak domestic demand in China due to virus lockdowns and the lure of better prices overseas prompted some producers to ship the product for the first time last month. PetroChina Dalian exported 3,500 tons, while Sinochem Quanzhou shipped 1,800 tons to Singapore, according to separate statements.

The free-on-board price for MTBE in Rotterdam-Amsterdam was about $530 a ton higher than that in China as of the end of May, according to ICIS. Chinese exporters could make a windfall of 2,000 yuan ($299) a ton in May and June, OilChem estimated.

A rebound in Chinese fuel demand as the nation returns from virus restrictions is likely to lead to reduced MTBE flows, according to traders and executives at Chinese refiners, who asked not to be identified because the information is private. OilChem estimated July exports will fall to 50,000-to-60,000 tons.