A lightweight portable, modular, and undiluted Hg measurement system satisfying EPA Method 30A requirements was developed by RSE and the ERC. The prototype hardware developed can be used for conducting RATAs according to the EPA Method 30A (including stack traversing), and as a portable Hg measurement system. The most important part of the system is a sampling probe, which was designed to allow for system calibration in the full calibration (flooded probe) and standard addition (dynamic spiking) modes. The specially designed heated probe head, manufactured of titanium, allows introduction of the barrier and calibration (spike) gases through ports integrated in the probe head. Probe body (extension) is manufactured of stainless steel to reduce cost. Following development and laboratory testing, the system was field-tested in four test campaigns conducted in Italy in November and December 2008, November 2009, and June 2010 at Monfalcone Unit 1 (160 MW coal-fired unit equipped with the ESP and wet FGD for particulate and SO2 control), and in the US in July 2010 at the 600 MW lignitefired partially scrubbed unit. Results of these tests including system performance are described in this paper.