Beauty pageant organisers are investigating who doused a contestant's evening gowns with pepper spray and spiked her makeup, causing her to break out in hives.

Beauty queen Ingrid Marie Rivera beat 29 rivals to become Puerto Rico's 2008 Miss Universe contestant, despite applying makeup and wearing evening gowns that had been coated with pepper spray, pageant spokesman Harold Rosario said.

Rivera was composed while appearing before cameras and judges throughout the competition. But once backstage, she had to strip off her clothes and apply ice bags to her face and body, which swelled and broke out in hives twice.