A study was undertaken to determine the effects of air pollution and precipitation on the 14th-century Our Lady Basilica in Tongeren, Belgium. The original stone used in this building, Maastricht limestone, is the typical building stone of Romanesque and Gothic monuments in the Limburg province. It contains 90 to 98 percent lime, and is mainly composed of microfossils. Samples of the deteriorated layer were taken at different orientations. Thin sections were made perpendicularly to the weathering zone and thoroughly analyzed petrographically. Additionally, depth profiles of the weathering zone were mapped by electron probe x-ray microanalysis. The morphological structure was also studied. Detailed quantitative analyses were carried out on crust samples using various multielement techniques such as energy-dispersive x-ray fluorescence analysis, ion chromatography, and atomic absorption spectrometry. Fourier-transform infrared spectroscopy with photoacoustic detection was applied to small pieces of the crust. On the North side, extensive growth of microorganisms occurs. On the South side algae grow a few millimeters behind the outer layer. A dark surface crust is formed at parts protected from rainfall, through sulfation and atmospheric particulate pollution. Parts flushed with rainwater have a grey coherent outer layer, a lower sulfate concentration compared with black crusts, and a compact structure due to recrystallized calcite. Especially at roundings, the outer black layers become detached from the unaffected material. Formation of gypsum is also found to a greater extent inside the stone. -- AATA