Coring of the Mariana Limestone on the Orote Peninsula of western Guam has created an opportunity to examine in detail the porosity evolution of a Pleistocene reef complex. Four cores acquired for study are composed predominantly of skeletal packstones, grainstones, and rudstones, with only minor bound-stone development. Deposition occurred within the reef-flat to proximal back-reef sand apron facies. Considerable primary interparticle and intraparticle porosity has been occluded by multiple-generations of submarine cementation and internal sediment infilling within the marine environment. In some instances, multigenera-tion, radiaxial to radial fibrous, Mg-calcite, isopachous rim cements have completely occluded all primary interparticle porosity within a Halimeda/coral rud-stone to Halimeda grainstone microfacies. Further porosity reduction has resulted from the precipitation of blocky and dog-tooth sparry calcite cement under freshwater meteoric conditions. Minor, secondary biomoldic porosity developed from the preferential dissolution of aragonitic Halimeda plates and some coral and molluscan debris within the meteoric realm. Study of the Mariana Limestone indicates that considerable porosity reduction may occur within the marine environment of many reef complexes. Only through significant leaching, dolomiti-zation, and fracturing, can such complexes be rendered porous.