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The phase behavior of quaternary microemulsion systems with a technical-grade nonionic surfactant (C8/10 alkyl polyglucoside) and different medium-chain alcohols (including the monoterpene geraniol) as cosurfactants is studied at 25 °C. Contour diagrams of the three-phase body at constant oil-to-water ratio have been mapped in terms of the mass fraction variables γ (surfactant + alcohol) and δ (relative amount of alcohol in the surfactant + alcohol mixture). Significant differences in the phase behavior of systems based on the pure alkyl glucoside β-C8G1 and those with the technical-grade C8/10 APG are observed. Specifically, at high δ an anomalous shape of the three-phase region, or a second narrow three-phase region appears in systems with C8/10 APG, which is absent in systems based on β-C8G1. Whereas at δ=0.5 all systems exhibit a phase inversion 2-3- $$\bar 2$$ on increasing γ, a retrograde transition 2-3-2 is observed at high δ. The anomalous shape of the three-phase body can lead to a phase sequence 2-3- $$\bar 2$$ -3-2* with a reentrant 2* state, on increasing δ at constant γ. It is argued that the reentrant 2* state is due to a preferential extraction of the more lipophilic oligomers of the technical-grade surfactant into the oil phase, causing the interfacial film to become more hydrophilic again, as δ is increased. [ABSTRACT FROM AUTHOR]