A validation of the ISO 787-23 method for measuring true density on reference black carbon samples and combustion emitted particles is proposed and compared to alternative methods. New measurements of true density on samples representative of fire emissions at different scales are presented and discussed, according to their respective experimental uncertainties. These results are compared to the values of soot true densities reported in the literature. An interesting correlation between the true density of combustion emitted particles and the Organic Carbon (OC) content is observed and commented: the true density significantly decreases as OC content increases. Three different ranges of true densities are then proposed as a function of the OC/Total Carbon (TC) ratio of combustion emitted particles. For low OC contents (i.e. below 5%) and high OC contents (i.e. above 20%), respective mean values of 1834 ± 187 kg/m3 and 1285 ± 217 kg/m3 are proposed. For intermediate OC content values, a fit is applied based on a mixing law. Finally, there is a discussion on the relevance of using these values for true densities of combustion emitted particles for temperatures representative of those reported in industrial fires, ranging from 25 °C to 240 °C.