The lateral instability of macrosteps has been identified theoretically as a mechanism whereby transverse modulations along a propagating macrostep can be amplified leading to the formation of solution inclusions. In this paper, experimental evidence for the occurrence of the lateral instability of macrosteps is presented. Ex situ images obtained by both optical and interferometric microscopy of the surface structure of several, {1 0 1} faces of a rapidly grown KH2PO4 (KDP) crystal are shown. The measurements give quantitative details of both the macrostep and the lateral instability structure. The observed wavelength of the instability is shown to be consistent with predictions from theory. Additional high-resolution surface profilometry measurements of the distribution of surface slopes of these faces show that there is a strong correlation between regions of locally increased vicinal slope and enhanced morphological instability. This is also shown to be consistent with theoretical predictions, and helps to explain the observed location of the surface instability.