Acid preparation of large vertebrate fossils poses special problems for the preparator. The Fundaci�n Colombiana de Geobiolog�a has prepared a number of large vertebrates (marine reptiles from the Cretaceous of Colombia, South America) using acid to remove calcareous matrix. A combination of factors, including: specimen size: choice of acid; number and length of acid baths; ventilation needs; area of matrix and fossil exposed; matrix homogeneity; number of acid resistant protective coats applied; management of voids; and acid consumption are shown to be important. By varying these parameters, exceptional preparation of specimens ready for detailed research and study can result.