The release of phosphorus from sediments can stimulate algal blooms in eutrophic water bodies worldwide. One technique to reduce this release involves capping the sediment with millimetre-thick layers of chemically active materials such as aluminium hydroxide floc (alum), and the mineral-based products Phoslock and modified zeolite. The effects of this technique on transport and reaction of diagenetically important sediment compounds other than phosphorus are unknown. The present study used microelectrodes to measure the apparent gas diffusivity of capping layers derived from different doses of these capping materials and their effects on pore water pH and dissolved molecular oxygen. The apparent O₂ diffusivity of alum capping layers (1.58 × 10^–5 cm² s^–1) was constant with depth and higher than that of mineral-based capping layers (~1.15 × 10^–5 cm² s^–1 and decreasing with depth in the capping layer). The capping materials raised the depth of the oxic–anoxic interface and associated pH minimum and altered the sediment O₂ consumption as functions of the capping-layer thickness and apparent diffusivity. Modified zeolite layers decreased pore water pH slightly (0.3–0.5 units); alum layers decreased pH by 1–2.2 units. It is proposed that capping layers derived from doses >200 g m^–2 can alter benthic process rates and solute fluxes.