In this paper we discuss the influence of chemical structures of renewable feedstock oils (RFOs) on the domains of existence and the nano-structures of microemulsions. We compare the results to those of classical microemulsions containing classical n-alkanes. First, the domains of microemulsions obtained from the melt of water, sodium dodecyl sulfate (SDS) as surfactant, 1-pentanol as co-surfactant and different RFOs (or RFO melts) in pseudo-ternary phase diagrams are presented. A surfactant–co-surfactant mass ratio of 1 : 2 is kept constant and the RFO (or RFO melt) is considered as a pseudo-constituent. Two different fatty methyl ester (FAME) biodiesels from rapeseed and cuphea oils, rapeseed oil, “TBK” biodiesel from rapeseed oil, limonene, and different mixtures of limonene to FAME-rapeseed biodiesel and FAME-rapeseed biodiesel to FAME-cuphea biodiesel are used as RFOs or RFO melts. Second, conductivity data are shown along an experimental path having a constant RFO or RFO melt : (surfactant–co-surfactant) mass ratio, whereas the water content is varied. All obtained data are then compared to data from previous studies with a series of n-alkanes (from n-hexane to n-hexadecane). As the main conclusion it is found that RFOs or RFO melts can easily substitute n-alkanes. From the chemical structure of the oils, it appears that not only the polarity of the oil plays an important role but also does the absolute size of the oil molecules. In all cases microemulsion systems exhibit percolative behavior.