The stocks of some of China's biggest companies are set to join a key global index by the close Thursday, giving ordinary U.S. investors greater access to mainland China.

More than $1.9 trillion in assets is benchmarked to MSCI's Emerging Markets index, which is tracked by the EEM ETF. The index has so far excluded hard-to-access mainland Chinese-traded stocks known as A shares and only includes Chinese stocks traded in Hong Kong or the U.S.

But by Thursday's close, around 230 of the A shares are scheduled to be included, kicking off a process that could eventually give A shares 16 percent allocation in the MSCI Emerging Markets Index and China a 42 percent weighting.

"This is a really important event that could change the face of EM investing," Sebastien Lieblich, global head of index management research for MSCI, said in a call with media Thursday morning New York time.

Emerging markets includes countries such as China, India and Brazil that tend to be growing rapidly towards a developed economy. For U.S. investors looking to invest in those foreign stock markets, U.S.-traded ETFs such as BlackRock's iShares MSCI Emerging Markets ETF (EEM) can offer access.