This paper examines the main results of an accelerated carbonation treatment applied to different types and size fractions of stainless steel slag. The objectives of this work were essentially to assess the CO2 uptake achievable by each type of slag under mild operating conditions and to investigate the effects of carbonation on the mineralogy and leaching behaviour of the residues. The following types of materials were tested: different size fractions of commingled slag, milled electric arc furnace (EAF) slag and argon oxygen decarburization (AOD) slag. Each material was thoroughly characterized in terms of elemental composition, mineralogy and leaching behaviour. Accelerated carbonation batch experiments were performed exposing humidified (with liquid to solid ratios <0.6 l/kg) slag to 100% CO2 for operating times between 0.5 and 24 h, at controlled temperature and pressure. Maximum CO2 uptakes of 130, 180 and 300 g CO2/kg slag were achieved (at 50°C and 3 bar) for the finest fraction of the mixture, the milled EAF slag and the AOD slag, respectively. The mineralogy of each type of residue showed to be affected by the treatment, exhibiting an increase in calcite concentration and a decrease in the content of specific silicate and oxide phases. The leaching behaviour of all types of carbonated slag was also modified, exhibiting a reduction by 1–2 units of the natural pH of the materials, accompanied by a decrease of Ca release and an increase of Si leaching, as a result of modified leaching-controlling phases. In conclusion, at the tested operating conditions, AOD slag was the most reactive material with CO2. Milling, however, proved effective in increasing the carbonation yield of the EAF slag compared to that measured for the different size fractions of the commingled slag mixture.