PARIS (Reuters) - A mixed set of rules internationally and low fines in some countries mean that bribery often pays off for companies even when they get caught, inter-governmental think-tank, the OECD, said on Thursday.

The Organisation for Economic Cooperation and Development said punishments for bribery varied widely among the 41 signatories of its anti-bribery convention, which aims to make laws consistent among developed countries.

Some countries have a maximum fine as low as $580,000 while others set a limit as high as $10 million and eight have no upper limit at all, the Paris-based OECD's 2016 Business and Finance Outlook said.

Using cash-flow simulations, the OECD calculated that 23 countries' maximum fines were not high enough to offset the financial return on investments in which bribery is involved.

"Sometimes sanctions are so light that even if people have a 100 percent chance of getting caught they would still choose to pay the fine and get the benefit of the act of bribery," OECD Secretary General Angel Gurria told a conference.

However, higher fines alone would not be enough to deter bribery because regulations are often poorly enforced. The three countries with the most punitive fines, which were not identified in the report, had not successfully prosecuted any company for bribery.

Enforcement data from members of the convention show that 24 states have not sanctioned a single individual or company for foreign bribery since it entered into force in 1999.

At the other end of the spectrum, Germany and the United States are the most active enforcers of bribery sanctions on both executives and firms.

(Reporting by Leigh Thomas; Editing by Paul Taylor and Louise Ireland)