Apple Inc on Tuesday surpassed Exxon Mobile Corp, if only briefly, to become the most valuable company in the United States.

After days of fickle stock market action, the iPhone and iPad maker's market value went up to touch $341.5 billion, while Exxon's market value stood at $341.4 billion. However, the oil major, whose annual revenue is four times that of Apple's, quickly got back to the top as its shares went up.

While Exxon saw its shares rising, Apple experienced some drops since global stock markets remained volatile due to soft economic data and the decline of the United States' sovereign credit on Friday. At 1:50 p.m. EDT (1750 GMT), Exxon's market cap stood at $339.3 billion, while Apple's plunged to $338.8 billion, Reuters reported.

Even if for a short while, Apple dethroned Exxon and joined a small league of companies that have experienced being on top in the S&P 500. The companies include General Electronics, General Motors, IBM, Microsoft Corp and AT&T, said Standard & Poor's Index Analytics.

Imminent iPhone 5 Launch

While Apple's market cap has soared by over $20 billion since July 1, volatile crude oil prices hit Exxon's market cap. During the same period, the oil major's market cap dipped almost $60 billion.

Market analysts said Apple's valuation has been fueled by the buoyancy that the upcoming iPhone 5, the new version of its best-selling phone, will lead to a monstrous increase of sales in the second half of 2011. The Cupertino-based tech giant has planned to sell as many as 25 million iPhones by the end of this year, according to reports.

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With a market cap of almost $342 billion, Apple's sales have been increasing 80 percent a year, coupled with even faster profit. For the fiscal year ending September 2012, the company is trading at about 11 times estimated earnings, while the S&P 500 index is valued at about 10 times next year's earnings. Apple also has $76 billion of cash and investments, a Reuters report said.

Robert Cyran, Reuter's Breakingviews columnist, has given an analysis of Apple's PEG ratio, which suggests the price of growth by dividing a company's PE ratio by its projected percentage of earnings growth. If the figure is small, it means that the company is cheaper. With that calculation, Apple's is 0.2.

According to Cyran, if Apple is put on the same PE multiple it traded on in 2006, the company's valuation would touch $900 billion, and with a premium for the current faster growth, the company could get as much as $1 trillion.

Compared to 2007 fiscal, Apple now sells more in each quarter. In addition, Apple's current return on equity is nearly double the size of what it was in 2006. That means that the iPhone maker has good pricing power.

Although growth could be slow, the ever-green smartphone and tablet markets will keep the ball rolling for Apple. The remarkable customer loyalty is an added advantage.