Abstract This technical paper explores two solutions for arithmetization of computational integrity statements in STARKs, namely the algebraic intermediate representation, AIR, and its preprocessed variant, PAIR. The work then focuses on their soundness implications for Reed–Solomon proximity testing. It proceeds by presenting a comparative study of these methods, providing their theoretical foundations and deriving the degree bounds for low-degree proximity testing. The study shows that using PAIR increases the degree bound for Reed–Solomon proximity testing, which affects its soundness and complexity. However, the possibility of reducing the degree bound with multiple selector columns is also explored, namely by following an approach based on the decomposition of the selector values. Focusing on performance optimization, the work proceeds by qualitatively comparing computational demands of the components of both arithmetization methods, particularly their impact on the low-degree extensions. The paper concludes that, while PAIR might simplify constraint enforcement, it can be easily translated to AIR, and system testing with benchmarks is necessary to determine the application-specific superiority of either method. This work should provide insight into the strengths and limitations of each method, helping researchers and practitioners in the field of STARKs make informed design choices.