Commercial, ready mix stone repair mortars have often been applied in conservation practice for the restoration of natura! stone, brick and terracotta, over the last decades with varying success in terms of durability of the repair mortar itself and compatibility with the substrate. In this research, four commercial stone repair mortars, commonly used in conservation practice in the Netherlands, have been characterized in laboratory. The composition of the repair mortars was studied by means of microscopy on thin sections. The effect of curing was assessed by comparing specimens cured in laboratory (28 days) and under outdoor conditions (28 days and 1 year). The porosity and pore size distribution of the mortars were determined by Mercury Intrusion Porosimetry and related to the measured capillary water absorption and drying behaviour. The hygric dilation both at different RHs and after saturation with water was measured. The flexural and compressive strength of the mortars were assessed, as well as their pull-off strength. Finally, based on the measured properties an attempt was made to evaluate the compatibility of the mortars with two limestone substraten of different total porosity and pore size.