As energy and fuel security continue to be of increasing importance to economies around the world, countries are looking to move away from oil dependency. Energy solutions are being sought in the area of solar fuels to meet these growing needs. Concentrated solar thermochemical technology has the potential to directly convert sunlight into a useable, carbon-neutral liquid fuel that can be easily stored and integrated into our existing forms of energy demand such as transportation and heating fuels. Ongoing research performed by several groups at Sandia National Laboratories seeks to fundamentally understand the complex physics and chemistry occurring within a solar thermochemical reactor prototype named the CR5 [counter-rotating-ring receiver/reactor/recuperator]. The CR5 utilizes a stack of counter-rotating disks with metal oxide reactive material fins which are cycled through oxidation and reduction zones. The metal oxide is thus used to reduce H2O and CO2 into H2 and CO respectively, which can be combined using known processes to form a liquid fuel. The effectiveness of such a solar thermochemical reactor depends on its ability to efficiently integrate reduction and oxidation reactions, a solar receiver, thermal recuperation, and separation of end product gases. Efficient separation of end product gases within the reactor is of critical importance as without it, the crossover of gases occurs, which results in lower reduction rates, recombination of end product gases, and additional energy spent in downstream processes. A validation reactor model called the CR5v (v for validation) has been fabricated to validate numerical models of the reactor processes. Crossover testing is done without any chemical reactions (therefore with no O2 or H2 present), but rather by examining the flow of CO2 and Argon. This work presents experimental crossover for the CR5v reactor as a function of ring rotation speed, internal purge gas, and sweep gas to injection gas ratio. Initial crossover experimental results from the CR5v reactor suggest that crossover levels are largely not affected by ring rotation, center purge or injection gas/sweep gas ratio. Argon flow remained on average at a crossover value of 52 %, while CO2 crossover levels were on average around 8 %. The crossover flow in the system is thought to be dominated by the flow rates of the two pumps used in the system and to a lesser degree, the geometry of the system.