Porosity is possibly an important parameter with respect to leaching of constituents from cement monoliths. During its lifetime, the pore structure of cementitious matrices changes due to carbonation and leaching. This paper discusses the effects of both accelerated carbonation and continuous leaching on the porosity, and, conversely, how porosity affects leaching properties. Two sample types are investigated: a mortar with MSWI-bottom ash substituting the sand fraction and a cement paste with 30 wt% of the cement substituted by a flue gas cleaning residue. The samples have been intensively carbonated in a 20% CO2 atmosphere for up to 60 days and were subsequently leached. The porosity was investigated by mercury intrusion porosimetry.