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Thermosensitive liposomes (TSL) in combination with regional hyperthermia represent a powerful tool for tumor specific drug delivery. The objective of this study was to investigate the influence of vesicle size on the biophysical properties of TSL. TSL were composed of DPPC/DSPC/1,2-dipalmitoyl-sn-glycero-3-phosphoglyceroglycerol (DPPG2) 50:20:30 (mol/mol) (DPPG2-TSL) and DPPC/P-Lyso-PC/DSPE-PEG2000 90:10:4 (mol/mol) (PEG/Lyso-TSL) with encapsulated fluorescent dye carboxyfluorescein, anticancer drug doxorubicin or magnetic resonance contrast agent gadodiamide. Extrusion was performed with polycarbonate filters of distinct pore size to obtain TSL with different diameters (50 to 200 nm). Phase transition temperature (Tm) of the bilayer forming phospholipids was not influenced by vesicle size in the tested range. However, vesicle size had a major impact on in vitro content release properties of TSL in the investigated temperature range between 30 and 45 °C. Generally, vesicle size was inversely related to content release properties with increased content release rates for decreased vesicle sizes. Size dependency of content release properties varied between all tested formulations and DPPG2-TSL were generally less affected by size changes in the range of 100 to 150 nm as compared to PEG/Lyso-TSL. Independent from gadodiamide release, vesicle size influenced the signal intensity of DPPG2-TSL also at temperatures below Tm due to improved water exchange for smaller vesicles. Liposomes around 100 nm in size are routinely used in vivo, hence a quality control for TSL preparations is required prior to use. Even small changes in size or a wider size distribution might affect stability and release properties and thus yield in decreased efficacy or unwanted side effects of drug loaded TSL during in vivo applications.