Xenoliths are pieces from the surrounding bedrock, brought to the surface of the earth by host magma. On Gran Canaria, the largest island in the Canary Islands archipelago, strange xenoliths have been found. They are light in color, porous and very different from the basaltic magma carrying them. By studying petrological features and oxygen isotope content of the xenoliths, the focus of this report will be to investigate their origin. The minerals and texture of 14 samples were studied in thin section, and the δ18O–value was determined for 17 samples. The mineralogical composition of xenolith glass was examined by EPMa. The results show that the xenoliths are rich in silica rich glass, quartz and feldspars, but also have high calcium content. 9 of 14 xenoliths have textures and δ18O–values from 8.1 ‰ to 16.77 ‰, similar to sedimentary rocks. The remaining xenoliths are metamorphosed and exhibit altered phenocrysts, indicating they have been melted and recrystallized. The latter group also has extremely low δ18O–value, which could be explained by the effects of hydrothermal processes. Most likely all the xenoliths originate from the prevolcanic sedimentary deposits beneath Gran Canaria.