The dual-fuel (DF) diesel-natural gas (NG) combustion concept for automotive engine applications has received increased attention in the recent years. The reasons of such interest lie in the need to identify energy sources alternative to fossil oil fuels, valid and competitive from an environmental point of view. Gaseous fuels, such as NG and methane, are very promising and highly attractive to this aim because of their easy availability, widespread distribution infrastructure, low cost, and clean-burning qualities. With respect to conventional diesel fuel, in fact, NG and methane permit a significant reduction of CO, and are less prone to soot formation. Notwithstanding that the significant soot reduction offered by the DF engine combustion mode is a consolidated result, the compliance with future stringent regulations on engine particulate emissions will still require the adoption of after-treatment devices, such as the diesel particulate filter (DPF). Therefore, a complete characterization of the particle emissions in DF configuration appears of great interest for the design of a future DPF equipping DF engines. To this aim, an experimental research activity was performed on a multi-cylinder automotive engine operated in DF mode, with a fixed methane substitution ratio value. The tests were carried out in transient conditions, typical of the engine homologation cycles, comparing the engine performance and emissions in conventional diesel and DF combustion modes. The present paper is mainly focused on the results of the engine-out soot particle analysis, performed by means of online and offline measurement techniques. The diagnostic systems led to the characterization of emitted particles, in terms of the soot mass concentration, particle number (PN), particle size distribution function, and chemical/physical features. The use of methane in the DF engine showed relevant impact in terms of soot mass and PN reductions but not on the particle average size. On the other side, the combustion mode proved to be not influential on the collected soot reactivity as well as to show a negligible impact on the soot chemicophysical features.