Luanda, the capital of oil-rich but poverty-stricken Angola, is the world's most expensive city for expatriates, according to a survey by consulting firm Mercer.

Luanda knocked Tokyo, which ranked No. 1 in 2009, from the top spot as the most expensive city in the report, which uses New York as the base city. All cities were compared against the U.S. metropolis, in U.S. dollars.