Standard and Poor's downgraded ExxonMobil's credit rating from AAA to AA+ on Tuesday because of expectations of continuing low oil prices.

S&P told CNBC that it has had a AAA rating on Exxon since July 5, 1949. The move leaves only Microsoft and Johnson & Johnson with AAA ratings from S&P.



Shares of ExxonMobil were flat on the day.

In its announcement, S&P said that it expects Exxon's "credit measures, including free operating cash flow (FOCF) to debt and discretionary cash flow (DCF) to debt, will remain below [its] expectations for the 'AAA' rating through 2018." The ratings agency added that its outlook on Exxon is stable.

S&P added that the "company's debt level has more than doubled in recent years, reflecting high capital spending on major projects in a high commodity price environment and dividends and share repurchases that substantially exceeded internally generated cash flow."