In this paper, the influence of cement kiln by-pass dust on the properties of Portland cement and slag cement was studied. The reference cements used in the experiment were CEM I 42.5R and CEM III/A 42.5N. The cement kiln by-pass dust replacement of cement was 0.7% and 1.7%, to satisfy the Cl- ions content in cements: lower than or equal to 0.1%, following the demand of PN-EN 197-1:2012 standard. The following properties of cements were examined: the heat of hydration, water demand for normal consistency, initial setting time and compressive strength. The results showed that the addition of cement kiln by-pass dust of up to 1.7% has no negative effect on the properties of Portland cement and slag cement. The cements represent the same strength class, as without dust addition.