Cross-sectional scanning tunneling microscopy (STM) has been used to characterize the atomic-scale structure of InAs0.35P0.65/InP and InN0.01As0.35P0.64/InP strained-layer multiple quantum well structures grown by gas-source molecular-beam epitaxy. Atomically resolved STM images of the (110) cross-sectional plane reveal nanoscale clustering within the InAsxP1-x alloy layers, with the boundaries between As-rich and P-rich regions in the alloy layers appearing to be preferentially oriented along the [(1) over bar 12] and [1 (1) over bar 2] directions in the (110) plane. (1 (1) over bar 0) cross-sectional images reveal that considerably less compositional variation appears within the (1 (1) over bar 0) plane; features elongated along the [110] direction are observed, but few [112] boundaries are seen. These observations suggest that the boundaries between As-rich and P-rich clusters may form preferentially within the (1 (1) over bar 1) and ((1) over bar 11) planes. Comparisons of filled-state images of InAsxP1-x/InP and InNxAsyP1-x-y/InP heterostructures suggest that N incorporation increases the valence-band offset in InNxAsyP1-x-y/InP compared to that in InAsxP1-x/InP.