This study examined the influence of powder composition and morphology on the penetration of Gray and White ProRoot mineral trioxide aggregate (GMTA, WMTA) and calcium hydroxide (CH) into open dentin tubules. GMTA, WMTA, and CH particle dimensions were analyzed by flow particle image analysis (FPIA). Penetration of open dentin tubules into dentin discs was studied by scanning electron microscopy. Five samples of each material were randomly selected and prepared for this study. The GMTA averages for length (μm), width (μm), perimeter (μm), and aspect ratio were 1.94 ± 1.65, 1.43 ± 1.19, 5.61 ± 4.27, and 0.76 ± 0.14, respectively. Corresponding averages for WMTA were 2.04 ± 1.87, 1.49 ± 1.33, 5.88 ± 4.81, and 0.76 ± 0.14, and for CH were 2.26 ± 1.99, 1.62 ± 1.46, 6.70 ± 5.60, and 0.74 ± 0.15, respectively. The rank order of the averages for particle length, width and perimeter from the largest to the smallest material was CH > WMTA > GMTA. The rank order of the averaged aspect ratios was GMTA > WMTA > CH. SEM showed that all three materials, when deposited and agitated on dentin discs, penetrated the open dentin tubules. Tubule occlusion occurred as particle surface concentrations increased. Significant differences in particle length, width, perimeter, and aspect ratio were observed for GMTA, WMTA, and CH (P < 0.0001 in all cases). All particle types penetrated into open tubules when agitated on dentin discs; all tubules were eventually occluded as particle concentrations grew. (J Oral Sci 56, 287-293, 2014).