Pena National Palace (Sintra, Portugal) was built in the 19th century and tiles, typical of Portuguese and Oriental art expression, were widely used as decorative elements throughout the outside walls of the palace. This study focuses on a passageway (Triton tunnel) that is covered with distinct tile panels from the second half of the 19th century, attributed to Wenceslau Cifka. Unfortunately, at present these magnificent tiles are covered by a green and/or black patina caused by microorganisms, whose identification was carried out by culture and molecular methods, light microscopy, confocal laser scanning microscopy, and scanning electron microscopy. Our results showed that the biological patina is composed of microalgae, cyanobacteria, bacteria, and some lichenized fungi. Some of these microorganisms penetrate within fissures and pores, producing tile biodeterioration. (c) 2012 Elsevier Ltd. All rights reserved.