An x-ray fluorescence measurement system has been developed with an 125I source to detect arsenic in superficial layers of phantoms and tissue. Based on in vivo measurements, in conjunction with Monte Carlo simulations, the detection limit for arsenic in skin ranges between 2.6+/-0.5 and 5.7+/-1.1 microg g(-1), depending on skin thickness and assuming that arsenic is uniformly distributed in the skin. The effect of skin arsenic distribution was also examined.