Petrographic and structural analyses of the pillow lavas in the Cumbres-Mayores spilitic complex show that they belong to two different basic volcanic rock types representing different conditions of emplacement. The first type exhibits decreasing crystallinity toward the center of the mass, radial disposition of amygdules, a clastic envelope, and a clay-sand cement between the pillows. This suggests that this type of pillow structure was formed in an unconsolidated mud. The second type has fluid structures parallel to the contours of the pillows, ovoid lithophysae, clastic aureoles and very little or no cement. These pillows were probably formed in open water, rolled on the sea floor, and massed in piles.