Nanometer-scale compositional structure in InAsxP1-x/InNyAsxP1-x-y //InP heterostructures grown by gas-source molecular beam epitaxy and in InAs1-xPx/InAs1-ySby/InAs heterostructures grown by metalorganic chemical vapor deposition has been characterized using cross-sectional scanning tunneling microscopy. InAsxP1-x alloy layers are found to contain As-rich and P-rich clusters with boundaries formed preferentially within ((1) over bar 11) and (1(1) over bar 1) crystal planes. Similar compositional clustering is observed within InNyAsxP1-x-y alloy layers. Imaging of InAs1-xPx/InAs1-xSby superlattices reveals nanometer-scale clustering within both the InAs1-xPx and InAs1-ySby alloy layers, with preferential alignment of compositional features in the [<(1)over bar 12>] direction. Instances are observed of compositional features correlated across a heterojunction interface, with regions whose composition corresponds to a smaller unstrained lattice constant relative to the surrounding alloy material appearing to propagate across the interface.