Sediment permeability is hypothesized to affect soft tissue fossilization within concretions through its effects on organic decay and concretion growth. The role of permeability was tested in a series of experiments in which cod tissue was decayed in glass beads of varying permeabilities. Decay was measured using infrared gas analysis (IRGA), and mineral precipitation within the beads (a proxy for concretion growth) was measured using micro CT scanning. The interactions of the three variables—sediment permeability, decay, and mineralization—were assessed with MANOVA and with linear regressions of decay and precipitation per unit decay on permeability. These two linear regressions were combined into a more general, nonlinear expression of the relationship between permeability and total mineral precipitation. The results show that sediment permeability has two competing effects on precipitation, the strength of each varying dynamically depending on permeability. Low permeability environments inhibit decay, thus enhancing fossilization but inhibiting overall precipitation because a build-up of decay products is necessary to promote mineral formation (the “decay effect”). However, low permeability environments can also increase precipitation per unit decay by inhibiting the diffusion of decay products away from the carcass, allowing for a faster build-up of decay products (the “mineralization effect”). At low permeabilities, the decay effect dominates (decay controlled), and precipitation is positively correlated with permeability. At higher permeabilities, the mineralization effect dominates (permeability controlled), and precipitation is negatively correlated with permeability. The experiments show that fossilization within concretions is promoted by decay inhibition (at low permeabilities) and rapid concretion growth (at intermediate permeabilities). Thus, the effects of permeability on fossilization are complex, and influence the mechanism of fossilization.