Average primary particle size (dp) and projected-area-equivalent diameter (da) of soot aggregates from two different sources and 16 fuels are measured from transmission electron microscope (TEM) images then the relationship between dp and da was fit with a linear regression in logarithmic space. The first source was a dual-fuel marine engine run on liquefied natural gas (LNG) with a diesel pilot and in diesel-only mode. The second source was a lab-scale turbulent diffusion flame run with 14 different fuel mixtures which are representative of global gas flaring fuels. Regardless of fuel type, gas flare dp, da relationships were very similar to the “universal fit” proposed in literature and each other. Conversely, the two fuelling conditions for the dual-fuel engine showed statistically significant differences not only in comparison to the soot from flares but also between LNG-mode and diesel-only mode.