This study evaluated interactive effects of mineralogy and organic matter content on chemical and physical properties of clay soils. Measurements were taken at different depths in four soil profiles characterized by oxidic, kaolinitic, mixed and montmorillonitic clay mineralogies, respectively. Within a given profile, organic carbon content varied more or less continuously with depth, whereas texture and mineralogy remained relatively constant. Thus in this study the combined effects of organic matter content and clay mineralogy could be evaluated in various combinations while texture remained constant. Resistance of soil aggregates to slaking by water was related primarily to soil organic matter content, with relatively minor differences attributable to mineralogy except in the oxidic soil where oxides seemed to exert an important stabilizing effect. Organic matter was associated with an increase in water-holding capacity both in the interaggregate pore space (0 to -0.33 bar moisture retention range) and intraaggregate porosity (-0.33 to -0.8 bar retention range). The enhancing effect of organic matter on intra-aggregate porosity seemed most prominent in the soils with oxidic and kaolinitic mineralogies, suggesting the importance of organic matter for maximizing plant-available water retention in these soils. For all soils and depths, pore-size distributions within the 0 to -0.33 bar moisture retention range were log-normally distributed, with the geometric mean pore diameter and log standard deviation practically constant. These results implied that although the total 0 to -0.33 bar porosity varied strongly (primarily with organic matter content), the relative pore size distributions were all similar. In agreement with other studies in the literature, effective cation exchange capacity (ECEC) increased and the zero point of charge (ZPC) of the variable charge complex decreased as organic matter increased, and as clay mineralogy varied in the order oxidic -> kaofinitic -> mixed -> montmorillonitic. Results of this study highlight the importance of managing organic matter for optimizing physical and chemical properties in soils, particularly those dominated by variable charge minerals.