A first part of the article deals with ambient carbon dioxide in the urban environment. A continuous measurement of this greenhouse gas has started at the Brno University of Technology in April 1999. The graphs show temporal and spatial variability of CO2 concentration values measured in Brno (April 1999 – August 2003). Building constructions in urban environment are negatively influenced by carbon dioxide in atmosphere and in rainwater as well. Evaluation of aggressive effect of rainwater caused by carbon dioxide is based on calcium – carbonate equilibrium. At five localities in the city of Brno and its vicinity the rainwater was sampled and its aggressivity has been evaluated using Heyer’s test. The results of the test showed that rainwater in this urban environment during summer period (April – August 2003) did not have acid character and was aggressive.