Abstract Tertiary-Holocene continental volcanic sediments in southern Guatemala were deposited in three basins each approximately 150 km long and 50 to 100 km wide. Boundaries between these basins mark the positions of transverse breaks in the underlying lithospheric slab. The rates of sediment accumulation and subsidence, and the composition, texture, and thickness of sediments can be expected to vary greatly from basin to basin. Exploration for hydrocarbon reservoirs in similar fore-arc areas should take into account that large scale correlations along depositional strike may be virtually impossible over distances greater than 50 or 100 km. The nature, degree, and timing of the diagenesis of Guatemalan volcaniclastics poses problems as to the hydrocarbon production potential of similar areas in other parts of the world. Guatemalan continental volcaniclastics are feldspathic litharenites. Three successive episodes of diagenesis have resulted in the precipitation of the following sequence of minerals in rock pores:—hematite-goethite;—montmorillonite plus hematite;—montmoril- lonite plus heulandite. These minerals occur as secondary pore-linings and pore-fills. Diagenesis has resulted in significant reduction of original permeability within 2000 years of deposition. Data indicate that the diagenesis of these rocks may not be dependent on depth of burial or temperature. The major diagenetic control was the chemistry of the groundwater. Introduction of groundwater resulted in the solution of unstable components (glass, pyroxene) and penecontemporaneous precipitation from pore-fluids of dissolved ionic species. Specific cements formed in the sands at any time are considered to be a function of the chemistry of the pore-fluids. Boundaries between different diagenetic assemblages in these rocks are probably determined more by ground water chemistry than by temperature or pressure.