BRUSSELS -- The European Commission opened an antitrust investigation into Oracle Corp.'s $7.4 billion acquisition of Sun Microsystems Inc., dealing a blow to Oracle's efforts to keep Sun customers who are increasingly being wooed by rival technology companies.

In the past few months, International Business Machines Corp., Hewlett-Packard Co. and Dell Inc. have all launched efforts to poach Sun customers. Those customers have been in limbo since April, when the Oracle deal was announced. Software maker Oracle has said publicly...