We present cross-sectional scanning tunneling microscopy (STM) studies of the atomic-scale structural and electronic properties of InAs/InAs1-xSbx superlattices grown by modulated molecular-beam epitaxy. In this technique the Group V composition is controlled by rapid modulation of the Group V beams rather than adjustment of the Group V flux ratios. A superlattice sample was grown at 475 degrees C consisting of 52 Angstrom InAs1-xSbx alternating with 172 Angstrom InAs for 30 periods on a GaSb (001) substrate. The InAs1-xSbx alloy layers consisted nominally of 7.8 Angstrom InAs alternating with 5.2 Angstrom InSb for four periods. X-ray diffraction was used to determine an average composition of InAs0.76Sb0.24 for the alloy layers. Constant-current STM images of the superlattice exhibit clear, electronically induced contrast between the InAs layers and InAs0.76Sb0.24 layers, and also reveal ordering within the InAs0.76Sb0.24 layers. Interfaces between the InAs layers and the InAs0.76Sb0.24 layers appear sharp, though there is evidence of more atomic intermixing at the InAs-on-InAs0.76Sb0.24 interface. Significant variation in composition within individual InAs0.76Sb0.24 alloy layers and apparent Sb incorporation from the InAs0.76Sb0.24 layers into the surrounding InAs layers are also observed.