Increasingly, soft processors are being considered for use within FPGA-based reliable computing systems. In an environment in which radiation is a concern, such as space, the logic and routing (configuration memory) of soft processors are sensitive to radiation effects, including single event upsets (SEUs). Thus, effective tools are needed to evaluate and estimate how sensitive the configuration memories of soft processors are in high-radiation environments. A high-speed FPGA fault injection system and methodology were created using the Xilinx Radiation Test Consortium's (XRTC's) Virtex-5 radiation test hardware to conduct exhaustive tests of the SEU sensitivity of a design within an FPGA's configuration memory. This tool was used to show that the sensitivity of the configuration memory of a soft processor depends on several variables, including its microarchitecture, its customizations and features, and the software instructions that are executed. The fault injection experiments described in this thesis were performed on five different soft processors, i.e., MicroBlaze, LEON3, Arm Cortex-M0 DesignStart, OpenRISC 1200, and PicoBlaze. Emphasis was placed on characterizing the sensitivity of the MicroBlaze soft processor and the dependence of the sensitivity on various modifications. Seven benchmarks were executed through the various experiments and used to determine the SEU sensitivity of the soft processor's configuration memory to the instructions that were executed. In this thesis, a wide variety of soft processor fault injection results are presented to show the differences in sensitivity between multiple soft processors and the software they run.