ACI Worldwide Inc., which makes software for managing electronic payments, said Tuesday that it has offered to buy financial services software provider S1 Corp. for about $540 million in cash and stock.

New York-based ACI's offer of $9.50 per share represents a 33 percent premium to S1's closing stock price of $7.13 on Monday and a 23 percent premium to the Norcross, Ga.-based company's 52-week high of $7.75.

The offer was made Tuesday in the form of a letter to S1's board from ACI's chief executive.

Under the ACI proposal, S1 shareholders could chose to receive cash or stock for their S1 shares, as long as 40 percent of the consideration is paid in ACI shares and 60 percent is paid in cash, ACI said.

Once the deal is completed, based on S1's current stock price, S1 shareholders would own about 15 percent of the combined company and ACI shareholders would own about 85 percent.

"With S1, we believe that ACI would further enhance its current position as a global leader in the enterprise payments software industry as a larger, more diversified company that is strongly positioned in a wide range of markets and supported by a broader base of revenues and earnings," ACI President and Chief Executive Philip Heasley said in a statement.

Heasley added that the addition of S1 would expand the services ACI can offer its customers and allow it to compete more effectively and profitably on a global basis.

ACI said it has secured financing from Wells Fargo Bank NA for the cash portion of the deal. If the offer is accepted, the sale could close as early as the fourth quarter, the company said.