SLN have been suggested for a broad range of applications, such as intravenous injection, peroral, or dermal administration. The incorporation of the drug in the core of the SLN has to be ensured for these applications, but the inclusion of drugs in SLN is poorly understood. This study is a contribution to further describe the inclusion properties of colloidal lipids and to propose incorporation mechanisms. Besides the well known methods to investigate entrapment of actives in nanoparticles such as DSC or microscopy, the present study focussed on yet a different approach. Based on the different chemical stability of retinoids in water and in a lipid phase, a method to derive information on the distribution of the drug between SLN-lipid and the water phase was established. Comparing different lipids, glyceryl behenate gave superior entrapment compared to tripalmitate, cetyl palmitate and solid paraffin. Comparing three different drugs, entrapment increased with decreasing polarity of the molecule (tretinoin < retinol < retinyl palmitate). The encapsulation efficacy was successfully enhanced by formulating SLN from mixtures of liquid and solid lipids. These particles were solid and provided better protection of the sensitive drugs than an emulsion. X-ray investigations revealed that good encapsulation correlated with a low degree of crystallinity and lattice defects. With highly ordered crystals, as in the case of cetyl palmitate, drug expulsion from the carrier was more pronounced.