Basalts and other mafic rocks are frequently used for road construction. The aggregates have to provide intrinsic mechanical strength and skid resistance. However, smectite contents stemming from alteration processes may deteriorate these mechanical properties considerably because of the resulting contrast in hardness. Quantitative relationships between smectite content, kind of distribution, and aggregate stability are rare except for a recent study by Kaufhold et al. (2012). In this work thirteen basalt and andesite samples from German quarries were analysed by X-ray diffraction (XRD) for their mineral contents (including clay minerals), their cation exchange capacities (CEC) and their resistance against disaggregation in dimethyl sulfoxide. Smectite contents from XRD exceeded those calculated from the CEC regardless of the kind of occurrence of smectite observed in thin sections. The hypothesis that rocks with smectites occurring mostly in patches should differ in terms of CEC and mechanical properties from those rocks where smectites are more disseminated could not be confirmed. However, the amounts of smectite accessible to cation exchange correlated with the DMSO losses. Therefore, the fast CEC determination with Cu-Trien exchange can be substituted for the time-consuming DMSO test.