Although it is not apparent on his financial disclosure form, Mitt Romney has millions of dollars of his personal wealth in investment funds set up in the Cayman Islands, a notorious Caribbean tax haven.

A spokesperson for the Romney campaign says Romney follows all tax laws and he would pay the same in taxes regardless of where the funds are based.

As the race for the Republican nomination heats up, Mitt Romney is finding it increasingly difficult to maintain a shroud of secrecy around the details about his vast personal wealth, including, as ABC News has discovered, his investment in funds located offshore and his ability to pay a lower tax rate.

"His personal finances are a poster child of what's wrong with the American tax system," said Jack Blum, a Washington lawyer who is an authority on tax enforcement and offshore banking.

On Tuesday, Romney disclosed that he has been paying a far lower percentage in taxes than most Americans, around 15 percent of his annual earnings. It has been Romney's Republican rivals who have driven the tax issue onto center stage. For weeks, Romney has cited a desire for privacy as his reason for not sharing his tax returns -- a gesture of transparency that is now expected from presidential contenders.