Cu2ZnSn(SxSe1−x)4 (CZTSSe) thin film solar cells have been prepared by vacuum-based thermal evaporation of metal and binary sulfide precursors followed by annealing in a mixed chalcogen vapor at 550 °C for one hour. The Zn/Sn ratio in the precursor was varied from 0.75 to 1.50 keeping the Cu/(Zn+Sn) ratio constant at 0.7. The best performing solar cell was obtained with a final film composition of Cu/(Zn+Sn)=0.77 and Zn/Sn=1.13 corresponding to a Zn/Sn ratio of 0.9 in the precursor. The champion cell exhibited an open-circuit voltage (VOC) of 506 mV, short-circuit current density (JSC) of 22.92 mA/cm2, and a fill factor (FF) of 35% resulting in a total area efficiency (η) of 4.06% without any antireflection coating. Cell performance was found to be limited by high series resistance (RS)=31.1 Ω and a low shunt resistance (Rsh)=125.2 Ω. No detrimental secondary phases, such as Cu2−xS(Se) or ZnS were detected in the absorber film. Microstructural investigation suggested that small multigrain structure of the CZTSSe absorber layer, presence of an interfacial Mo(S,Se)x blocking barrier, and micro-air-voids at the Mo back contact are the major contributors to the origin of high Rs. Morphological study of the CZTSSe film surface by atomic force microscopy revealed micro-pores that act as low resistance shunt paths and explains the source of such low Rsh. The performance limiting factors of the vacuum based thermally evaporated CZTSSe thin film solar cells are reported.