This chapter, authored by Robert C. Aller, focuses on the effects of macrofauna on the chemical properties of marine sediments and their overlying waters. The introduction highlights that the composition of marine sediments is significantly influenced by the movement of particles and fluids by large bottom-dwelling animals, which engage in activities such as feeding, burrowing, and tube construction. These biogenic processes have both direct and indirect impacts on the chemical and biological properties of the sediments. The chapter then delves into diagenetic reactions, which are crucial for understanding the chemical changes in sediments. These reactions, primarily associated with the decomposition of organic matter and other biogenic components like CaCO3 and SiO2–nH2O, affect pH and oxidation-reduction potential. The chapter provides detailed equations for several idealized decomposition reactions, including aerobic respiration, nitrate reduction, manganese reduction, iron reduction, sulfate reduction, methane production, and fermentation. These reactions influence the depletion or buildup of specific reactants and products in both the fluid and solid phases of the deposit, contributing to the overall chemical dynamics of the sedimentary environment.This chapter, authored by Robert C. Aller, focuses on the effects of macrofauna on the chemical properties of marine sediments and their overlying waters. The introduction highlights that the composition of marine sediments is significantly influenced by the movement of particles and fluids by large bottom-dwelling animals, which engage in activities such as feeding, burrowing, and tube construction. These biogenic processes have both direct and indirect impacts on the chemical and biological properties of the sediments. The chapter then delves into diagenetic reactions, which are crucial for understanding the chemical changes in sediments. These reactions, primarily associated with the decomposition of organic matter and other biogenic components like CaCO3 and SiO2–nH2O, affect pH and oxidation-reduction potential. The chapter provides detailed equations for several idealized decomposition reactions, including aerobic respiration, nitrate reduction, manganese reduction, iron reduction, sulfate reduction, methane production, and fermentation. These reactions influence the depletion or buildup of specific reactants and products in both the fluid and solid phases of the deposit, contributing to the overall chemical dynamics of the sedimentary environment.