Laser light scattering and thermophoretic sampling have been used to investigate particle formation in counterflow diffusion flames inhibited by iron pentacarbonyl Fe(CO)(5). Three CH4-O-2-N-2 reactant mixtures are investigated, with Fe(CO)5 added to the fuel or the oxidizer stream in each. Flame calculations that incorporate only gas-phase chemistry are used to assist in interpretation of the experimental results. In flames with the inhibitor added on the flame side of the stagnation plane, the region of particle formation overlaps with the region of high H-atom concentration, and particle formation may interfere with the inhibition chemistry. When the inhibitor is added on the non-flame side of the stagnation plane, significant condensation of metal or metal oxide particles is found, and implies that particles prevent active inhibiting species from reaching the region of high radical concentration. As the inhibitor loading increases, the maximum scattering cross section increases sharply, and the difference between the measured and predicted inhibition effect widens, suggesting that particle formation is the cause of the deviation. Laser-based particle size measurements and thermophoretic sampling in low strain rate flames show that the particles have diameters between 10 nm and 30 nm. Thermophoresis affects the nanoparticle distribution in the flames, in some cases causing particles to cross the stagnation plane. The scattering magnitude in the counterflow diffusion flames appears to be strongly dependent on the residence time, and relatively independent of the peak flame temperature.