Background: The strength characteristics of two genetically different shales treated with both Rice Husk Ash (RHA) and Coconut Husk Ash (CHA) was evaluated to elucidate responses and effects, examine effectiveness of the additives with a view to ultimately provide economically viable and environmental friendly options for modification and hence stabilization. Methods: 2 to 20 % by weight of both RHA and CHA were separately added to Okitipupa (SW) and Enugu (SE) shales with the subsequent determination of Plasticity Index (PI), Maximum Dry Density (MDD), Optimum Moisture Content (OMC), Unconfined Compressive Strength (UCS) and California Bearing Ratio (CBR). Results: RHA and CHA were found to possess pozolanic properties such that their addition to shale in modest amounts (not more than 10 % by weight) has beneficial effect on the strength characteristics. Addition of RHA produced shales with reduced PI, higher UCS, increased MDD and more pronounced reduction in OMC when compared with the CHA stabilized shales. However in general, addition of 10 % RHA and 6-10 % CHA brought about optimal effect on the geotechnical properties of shales and as such can be regarded as the optimum content. Conclusions: These materials can thus serve as suitable alternatives to modify and stabilize problematic shale and hence help reduce construction costs, environmental hazards and ultimately bring about shales with improved geotechnical properties.