The Alboran Sea is a tectonically active basin in the westernmost Mediterranean Sea created during the Neogene collision of the African and European plates. It has a complex history that does not clearly fit a previously defined tectonic style. During Ocean Drilling Program Leg 161, Pliocene to Miocene sand-bearing sequences were cored at four sites (Sites 976-979) in an east-west transect across the Alboran Sea. More than 150 sand samples from these cores were air-dried and sieved to obtain the sand-sized fraction. Thin sections prepared from these samples were stained for recognition of feldspars. Sand detrital modes were determined for 34 samples from these sites in addition to 10 samples of beach sand collected along the southern Spanish coast. A maximum of 400 points were counted using the Gazzi-Dickinson method. Site 976 is located at the base of a southward dipping slope on the Spanish Margin. At this site metamorphic basement is overlain by a Miocene volcaniclastic interval. Pliocene sand is predominantly quartzolithic, whereas Pliocene sand is more quartzose. Sites 977 and 978 are located to the south and north, respectively, of the Al-Mansour Seamount in the Eastern Alboran Basin. Pliocene sand at these sites is quartzose, whereas the Pleistocene sand is principally quartzolithic. Site 979 is located in the southern Alboran Basin, in a narrow depression between the Alboran Ridge and the Moroccan coast. Sand at this site is glauconitic with significant glassy volcanic components in the lowermost section. Mean detrital modes for the most sands in the Alboran Basin plot in the Recycled Orogenic and Magmatic Arc compositional fields. A few exceptions plot in the Dissected and Magmatic Arc fields. The quartzolithic composition and ratio of sedimentary to metamorphic lithic fragments in sand samples from Unit II at Site 976, Unit I at Sites 977 and 978, and Unit I at Site 979 reflect the proportion of metamorphic and sedimentary rock sequences exposed in the Betic Cordillera of southern Spain (Sites 976-978) and in the Rif in Northern Africa (Site 979). Analysis of coastal beach sand suggests that very quartzose sands from Unit II at Site 976 were likely derived from the Flysch Trough Units near Gibraltar. Upper Miocene samples from Site 976 contain a significant amount of volcanic debris. This sand composition is probably a result of widespread volcanic activity during basin inception and formation. In contrast, lower Pliocene volcanic sand from Site 977 is probably related to later intrabasinal volcanism.