There is no way of knowing in advance when a stock split of Exxon Mobil may occur. Publicly traded companies have a finite number of shares of stock available on the stock market, according to Investopedia. Stocks split when a corporation's board of directors determines that additional shares are needed.
In a two-for-one stock split, shareholders are given an additional share of stock for every share owned, notes Investopedia. Splits typically decrease the price of stocks, since more shares are available. Although the number of shares increases, total market value stays the same.
Companies typically issue a stock split when share prices increase to levels that are significantly higher than competitors, Investopedia notes. The primary motivation behind a stock split is to make shares appear more affordable to investors, according to Investopedia. As of 2014, Exxon Mobil stock last split in 2001, according to Morningstar.