The United Nations has criticised Thailand for deporting two people who had been given refugee status by the organisation.
The two are thought to be the Chinese dissidents, Jiang Yefei and Dong Guangping, who had fled to Thailand with their families.
The pair had previously been in jail in China for criticising the communist government.
Both were in detention in Thailand for illegally entering the country.
The UN Refugee Agency said: "This action by Thailand is clearly a serious disappointment and underscores the long-standing gap in Thai domestic law concerning ensuring appropriate treatment of persons with international protection need."
According to the rights group Amnesty International, Mr Jiang was detained for two brief periods in 2008 after complaining about the official response to the earthquake in Sichuan province that year.
He fled to Thailand shortly afterwards with his wife.
Mr Dong was released in February this year after spending 10 months in jail in China for taking part in an event to remember people killed following protests in Tiananmen Square in Beijing in 1989.
He arrived in Thailand with his wife and daughter in September.
Both Chinese dissidents, who were due to be settled in a third country by the UN, were recently fined for breaking Thai immigration rules.
Amnesty International said that while the two were in jail and waiting to be resettled abroad, their fines were paid by someone not known to either man. It appears they were then transferred to an immigration detention centre before being deported over the weekend.
Amnesty believes the two men have been returned to China and are "at risk of torture and other ill treatment".
Three other people are thought to have been deported with the two dissidents.
There is speculation that one of them is Hong Kong-based publisher Gui Minhai, who has not been heard from since arriving last month in the Thai resort of Pattaya, where he owns a holiday home.
Thailand faced criticism earlier this year when it sent back to China more than 100 Uighur Muslims, who had been detained as part of a larger group the previous year.
Many Uighurs complain of repression in the Chinese region of Xinjiang where they live, and at least some of those who arrived in Thailand had been trying to get to Turkey.
Thailand is currently governed by the military, which took power in a coup last year.