Apple Inc's market value climbed past US$623 billion (S$782 billion) on Monday, surpassing the record set by Microsoft Corp during the heyday of technology stocks in 1999.

Apple shares rose 2.6 per cent, bringing its gains this month to almost 9 per cent as Wall Street bets on the September 12 rollout of the latest version of the iPhone, the device that revolutionized the mobile industry.

Microsoft, however, retains the title of history's most valuable company if its 1999 peak value of about US$621 billion were to be adjusted for inflation.

Apple's stock usually rallies in the run-up to major product launches, among the most heavily watched events on the annual tech calendar. The iPhone is the company's biggest product, yielding half or more of its sales.

Sources have said the company will take the wraps off a larger version of its iPhone on September 12. Some analysts also think it intends to announce a smaller iPad to safeguard its market share, as rivals from Google Inc to Amazon.com Inc begin selling cheaper, seven-inch tablets.

But Bernstein Research's Toni Sacconaghi warned that questions remain about the availability of components for both the iPhone and the iPad, which in the past has constrained Apple's product shipments.

"A key question for the launch will be Apple's expected rollout schedule," the analyst wrote on Monday. "Apple's intention is to continue to ramp offerings as quickly as possible, but the company's ability to do so remains a key near-term question."

Apple's shares have risen 64 per cent in 2012. On Monday, they closed at a session high of US$665.15, conferring on the Silicon Valley giant a capitalisation of $623.5 billion, exceeding Microsoft's 1999 value of US$620.8 billion, according to data provided by S&P Dow Jones Indices.

But Microsoft's value would rise to US$853.7 billion after adjusting for rising prices, according to the Bureau of Labor Statistics' inflation-calculator.

Polar Opposites

Apple overtook Exxon Mobil to reach the No. 1 spot by market capitalisation last year.

Monday's move means it has now entered the record books as the biggest company ever, in terms of market value.

"Everyone loves a winner; if you play the quick trade be careful," said Howard Silverblatt, senior index analyst at S&P Dow Jones Indices in emailed comments. "If you are an investor, check the fundamentals and business plans, and avoid the hype in your decision."

Apple climbed even as fellow technology heavyweight Facebook Inc plumbed new depths. The No. 1 social network slid to a record intraday low of $18.75 in the morning before bouncing back to close just above US$20 after Capstone upgraded the company's stock to buy from hold.

Facebook's stock has gone south in the past month as investors worried about its ability to make revenue grow. Last week, some early investors were given the go-ahead to sell for the first time since Facebook's May 18 IPO.

Several similar lockups will expire through the end of the year.

Facebook rebounded above US$20 in afternoon trade after Capstone's upgrade, based on a combination of a more attractive valuation since its decline, and good long-term advertising prospects.

"It seems to be down around levels that people who didn't like the deal thought it was really worth. And now it seems to have stabilized," said Eric Kuby, chief investment officer, North Star Investment Management Corp in Chicago.

It may have "found a level which seems more of a better price for people valuing the company in terms of the future."