I am a US professor teaching in summer in China for less that 90 days. Am I liable for Chinese tax (automatically deducted from pay). My reading of article 19 of the US-China tax treaty of 1984 suggests that the such income is not taxable. IRS gives an example of a Chinese researcher that get income from US and states that the income is not taxable in the US. However, is the revers true?

The reverse IS true if you have the Chinese equivalent of a J-1 visa. With such a visa, your teaching income will be completely tax exempt.