ABSTRACT: This paper reports an investigation of the influence of pyroclastic rock dust on the geotechnical properties of expansive soil. The plasticity, linear shrinkage, compaction, California bearing ratio (CBR) and shear strength characteristics of the soil when mixed with varying proportions of pyroclastic rock dust were investigated. The results show significant reduction in plasticity and linear shrinkage of expansive soil with increasing amount of pyroclastic rock dust. The maximum dry density, optimum water content, shear strength and CBR all increased with increasing pyroclastic rock dust content. Optimum CBR values were obtained with the addition of up to 8% pyroclastic rock dust. The behavior of the geotechnical properties of the expansive soil when blended with pyroclastic rock dust indicates that the rock dust is a good modifier for this problem soil.