Co-firing NH3 with other fuels is receiving growing interest as a feasible solution to improve its combustion and emission properties. Previous studies mainly focused on NO, emission, and paid less attention to soot formation which is investigated here using a laminar counterflow flame configuration with ethylene fuel (75% by mole) mixed with different proportions of ammonia (between 0 and 25% by mole while the remainder is nitrogen). Soot volume fraction (SVF) was measured using planar laser induced incandescence (PLII). It was found that the addition of ammonia significantly reduced the measured SW by 4-6% per 1% ammonia addition as compared to the reference flame (25% nitrogen). To rule out temperature effects, the experiments were simulated using Chemkin Pro and it was found that there were negligible differences in temperature between each condition implying that temperature was not responsible for the reduction in SW. To investigate the chemical effects of ammonia addition, polycyclic aromatic hydrocarbons (PAH) were measured using planar laser induced fluorescence (PLIF) at 4 wavelengths (350 nm, 400 nm, 450 nm, and 500 nm). PLIF intensities at 350 nm is deemed to correlate with PAHs of 2-3 rings and measured profiles at this wavelength were nearly overlapping for all cases. These findings were supported by GC-MS measurements of acetylene and benzene with the latter showing little change in the peak for the cases studied here. At longer wavelengths, PLIF intensities began to show the same trends found for SVF measurements. Additionally, a specific nitrogen detector was used during GC-MS measurements and several nitrogen containing hydrocarbon species were detected with the 25% addition of ammonia. The combined results indicate that the nitrogen containing hydrocarbon species are likely to account for soot reduction, with the precise mechanism yet to be elucidated. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.