BUSINESSES should be wary of mainland companies and get promises in writing before committing to joint ventures, said a major accounting firm. Deloitte Touche Tohmatsu general manager in Shanghai Kenny Poon said: ''Some Hong Kong clients of mine have set up a vague joint venture with a Chinese company. ''They later found their partners didn't even own the factory and staff quarters they were shown and promised.'' He said many companies rushed into China to set up operations without first discussing specifics. His clients in joint venture had invested about $2 million. Chinese officials were investigating, Mr Poon said. Growing business interests in China meant having to learn about Chinese financial procedures and policies, said Connie Sit of Radica, a Hong Kong electronics company. ''It is an entirely different system than the one we're used to,'' said Ms Sit whose company will start a joint venture in Dongguan, in southern China. That task became harder when China changed its accounting regulations for foreign investment in 1992. Ms Sit has paid $7,900, along with 50 others, for a two-day seminar on China's joint-venture accounting regulations which started yesterday. The China Join-Venture Accounting course covers China's accounting framework as well as procedures for Hong Kong investors. The seminar is sponsored by China Law and Practice, a legal publishing company made up of lawyers. Lawyer Bridget Chi, also attending the seminar, wanted to get familiarised with the terms because her firm works on many joint-venture contracts in China. There was usually a section on accounting procedures and underlining concepts in a joint-venture contract, she said. ''Knowing about the recently changed regulations is helpful. These new rules affect our clients,'' said Ms Chi. Mr Poon was a speaker at yesterday's seminar session. ''In China, you must deal with local authorities directly. It's really dangerous to go over someone's head,'' Mr Poon said. He also said when there was a conflict between the tax law and accounting regulations, the law always took precedence. Mr Poon has experience in developing accounting systems in Beijing and Hong Kong. He has consulted with the Shenzhen Stock Exchange and the People's Bank of China on the effects of issuing Chinese companies' shares to foreign investors.